I agree that he is flexing his knees during the backswing. Biomechanically, he is still performing the same motion - bracing the knees by flexing the knees so that they are ready to withstand the downward arm swing thrust action movement without losing any body angles or balance. Theoretically, that knee flexing action could presumably occur in the early downswing - just before, and in anticipation of, the downward arm thrust action movement.
Here are images of VJ Singh where it appears that his knee flexing action occurs in the early downswing and not the backswing.
Not all PGA tour golfers seem to need that knee-flex action.
Here are a series of images of Adam Scott. His knee flex angle doesn't seem to change much during his backswing/downswing.
Position 1 and 2 = backswing, positions 3 and 4 and 5 = downswing.
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