Load the lead leg and push off the ground for power
Breaking down the swing sequence, Saguto shows that at the top of the backswing the lead (left) knee works down toward the ground as pressure and weight load into the lead leg. He says this both keeps the body centered over the ball and creates power, because the golfer can then push off that front leg in the downswing.
Channel: SagutoGolf
Watch official source at 1:56Why this context matters
Saguto is breaking down Tiger Woods' swing photos from a Golf Digest article.