Knee bend timing in "The serve, part 2"

[maverick1]

This article "The serve, part 2" in Classic Lessons says:
There's a simple formula to time the knee bend. Flex the knees down as the hands are going up. The knee bend should be fullest just after you release the toss.

That seemed too early to bend the knees.

Before posting, I checked the serves of Sampras and Roddick. Their knee bend barely starts as they release the toss. The fullest bend is around the time the ball reaches its peak.