Originally Posted by Mr_Super_Socks
you obviously don't need to get into that position while riding, but doing this stretch will give a pretty good indication of the range in your hip flexors. having flexibility there is what allows you to sit comfortably with your bars well below your seat, with your hips rotated forward and your legs still able to generate good power.
I can do the stretch to where my wrist is past my foot and I can even double over like the pro in Euro's pic, but I still have bars only about 2" below the seat.
Yeah comfort, or lack of discomfort is only one factor. There's a gray area where as the hip angle becomes more acute, there begins a loss of power generation without any noticable discomfort. The body has the resiliency to ride in that position, but the muscles are not yet adjusted to function optimally.