I didn't say it was bent on the down kick, just that it is bent when it is down. Presumably the upward motion begins earlier in the leg above the knee and then the lower leg follows, giving the whipping action. There is definitely a point at which the foot is much lower than the knee. It's most evident in underwater shots taken from in front of the swimmer but it can also be seen in side view underwater shots.
Slow motion or stop motion would be useful.
This passage from Swimming Fastest by Ernest Maglischo is interesting:
"...the beginning of the upbeat actually appears to be part of the preceding downbeat. Swimmers gently flex the leg at the knee as the foot passes below the body and then presses the thigh slightly upward to initiate the upbeat. At that time, the pressure of the water above the leg pushes the lower leg downward further into a flexed position while the thigh is actually moving upward. This gives the impression that the downbeat is still underway, but in actuality, it has ended and the upbeat has begun."
Possibly this is one of those things that happen naturally and don't require conscious thought.