When did the balance of power shift from the Powhatans to the English?
The Powhatan confederation was at its strongest relative to the English during the "starving time" of 1609-1610. Had the Powhatans pressed their advantage then, colonization might have been delayed, A different set of colonizers, other English, or perhaps the Spanish, French or Dutch, might have arisen instead in the Chesapeake area.
During the "Peace of Pocahontas" (roughly 1614- March 1622) hostilities were at a minimum, but the English advantage was increasing as more settlers arrived and made progress at the task of survival in North America.
The aftermath of the coordinated attack of March 1622 was the tipping point and the last opportunity for the Powhatans to regain control of their territory. They continued to fight, making opportunistic attacks on settlers who strayed from their forts. This was the style of fighting that made best use of their weaponry and talents. We might speculate that they failed to press their advantage forcefully enough, as happened during the starving time, but this is difficult to assess. There are numerous settler reports of Indian attacks, many of them successful. But since we only know the end result and the English side of the story, it's difficult to know precisely how, or whether the Powhatans could have made better choices to effect a different outcome.
A single moment that might serve as representative of the power shift from the Powhatans to the English is the 1624 battle at Pamunkey River, when the English and Pamunkeys met in a rare open battle. After the attack of 1622, the colonists had begun amassing antiquated chain mail armor shipped from England that was nevertheless effective against arrows. A force of over 800 Indians fought a two-day battle against 60 well-armed and armored English soldiers and militia, resulting in the deaths of numerous Indians and only a small number of English. The moment this battle ended might be as close as one can get to the passing of power from the Powhatans to the colonizers. The Indians continued to fight, but after that battle, they never seriously threatened the English hold on Virginia.
That's one way to look at it. It's equally likely that the moment power passed to the English went unnoticed and unrecorded. Perhaps it happened as a group of Powhatan Indians sat on a rise overlooking a river where boats with sails and oars had replaced the dugout canoes of their youth. Some of them had lost children to frightfully quick and lethal diseases. Others had lost limbs to weapons they barely understood. They may have looked at the river traffic and wondered how they fit in to this new and terrifying world that their priests and elders were at a loss to explain.
From James Horn, A Land as God Made It (2005).
"The decisive battle of the war took place the following year. In July 1624, Wyatt, with sixty men in armor, sailed up the Pamunkey River into the heartland of Opitchapam's territory, where they were confronted by approximately 800 bowmen and an unspecified number of allies "that cam[e] to assiste them." Encouraged by their own numbers, or the "paucytie of ours," as Wyatt commented, on this occasion, Pamunkey warriors took on the English "in open fielde" and fought with great determination to defend their villages and a great quantity of corn. They fought also to defend their reputation as the elite of Powhatan warriors, the "Pomunkeys having made greate braggs of what they would doe, Amonge the Northerne nationes." The battle continued for two days but, despite their valor, which even Wyatt acknowledged, the Pamunkeys were beaten back by the power of soldiers armed with snaphaunce muskets, an early form of flintlock that was far more effective than the old matchlock. Eventually, after sustaining heavy losses, the Indians "gave over fightinge, and dismayedly, stood most ruthfully [ruefully] looking on while theire Corne was cutt downe." The English, protected by their armor, suffered only light casualties. " p. 271 Citing Fausz and Rountree.
Helen Rountree, in Pocahontas, Powhatan, Opechancanough (2005) tells the story with some variation:
"This year  the Pamunkeys planted more corn than usual, so that if their allies on the James River lost theirs, they could make up the difference. Those plans proved to be futile.
Nepinough [harvest time] came on time, that year being a wetter-than-average year.  The cornfields that had been planted first were becoming ripe, and the later-planted ones were well grown; the crop looked promising. Because Opechancanough was still apparently very ill, Opitchapam ("Otiotan") was governing the people, and in a fit of bravado he had done some bragging. Not only would the Pamunkeys feed their allies if needed, but when the enemy raiders came, as they inevitably would, his men would give them a fight they would never forget. Word of that boast reached all the way to the Patuxent River in what is now Maryland, for the Patuxent chief actually sent an observer southward to see the fireworks.
Sure enough, the governor of the Tassantassas [the English] led sixty soldiers up the Pamunkey River to the mamanatowick's town. A huge force of warriors stood ready for them, if the governor can be believed; "eight hundred ['Pamunkey'] bowmen, besides divers nations that came to assist them," all being intent upon defending not only the winter's supply of corn but also their own reputation as fearsome fighters. Once ashore, two dozen of the enemy (presumably armed with swords) headed over to the cornfields to begin hacking down the plants. The other three dozen, armed with their guns, engaged the defenders who were trying to get their corn cutters to cease operations. The open-field skirmishing went on for two whole days, and by the end of it, so many of Opitchapam's men had been killed and so much corn had been cut ("sufficient to have sustained four thousand men for a twelvemonth") that the people quit struggling and let the enemy cut down the rest of the crop while they "stood most ruthfully looking on." It was a smashing defeat for the Pamunkeys.  No more raids on native towns took place that summer, but no more were needed. The people's corn was gone for that year, and their men's reputation was ruined for a long time to come. Opechancanough, wherever he was lying and trying to recover, must have been fuming helplessly." p. 221, 222. Citing Stable 1998 and Kingsbury 1906.
beaver range from "NatureWorks"
Beaver extermination, pig introduction and the loss of Native friendly habitat
When considering the transfer of territory from Indians to English, it is tempting to imagine only conquest and spread of disease as the overriding factors in how the colonists acquired Indian territory. Although those two processes had an enormous effect on the Indian population along the Eastern seaboard, one must also consider the massive and rapid changes in the environment that followed the arrival of the Europeans that impacted traditional Indian lifestyles and forced them further west. Daniel K. Richter's 2003 book, Facing East from Indian Country is my main source for the information on dramatic environmental change that follows.
One of the ways Europeans found Indians to be useful, besides providing them with corn, was their ability to procure furs for the European fur trade. The high value of beaver fur quickly led to regional extinctions. When beavers are eliminated from an ecosystem, the effects are wide-ranging. Beaver are natural woodcutters that clear the forest near streams. Their dams transform the streams into ponds and wetlands. These in turn, provide habitat for insects, fish and other wildlife that attract birds and small mammals, many of which were sources of food and raw materials for Native populations. Deer and bear also gathered at beaver ponds and wetlands. When beavers are eliminated from the ecosystem, the wetlands disappear, exposing naturally cleared land that provided rich meadows Europeans could quickly occupy for farmland. Wherever colonists located farms, the land available for deer and other game was reduced by the introduction of fences and livestock.
The introduction of pigs contributed further to environmental change. Pigs were often allowed to forage freely around farms. Indians were forbidden from hunting pigs, which was regarded as theft, but pigs could not be prevented from consuming Indian crops. They also ate the plants and shoots that would have been available for deer, resulting in reduction of game that Indians relied on. Wherever Europeans settled, the surrounding areas quickly became unsuitable for traditional Indian lifestyles.
"The arrival of European farmers--with their roaming livestock, their concepts of fixed property, and their single-crop plow agriculture--combined with the ecological impact of the fur trade to transform utterly the material environment of much of Eastern North America and make traditional patterns of life impossible anywhere in the vicinity of European settlements. European and Indian ways of using the land could no more share the same ecosystem than could matter and anti-matter share the same space." p. 59, Richter (2003), Facing East from Indian Country; a Native History of Early America.
"Beaver" with info on historical distribution (PDF), by Baker, B, and Hill, E., from Wild Mammals of North America: Biology, Management and Conservation National Geographic Beaver Documentary on YouTube ...