Olympic Lifters and knees forward

[Baldr]

How can olympic lifters lift with the knees traveling forward in the bottom of the squat/snatch/clean and jerk movement (like pg.45 in SSBBT) in an exaggerated fashion, and not injure themselves? I would especially think their hip flexor tendons would receive a beating. After watching the olympics videos on Youtube, it looks like their knees are as far forward as possible.

[Mark Rippetoe]

Their knees get adapted to this motion, just like anything else adapts. The real question is: why do they train their back squats this way when doing so omits the hamstrings?