Shoulder positioning when doing adductor flys

[davidbarrack]

In the video the adductor fly is shown when lying in to different positions. The first has the shoulders flat on the ground while the other shows Mr. Kurz lying against some padding which raise the upper back and shoulders. Does it matter which position is used? I prefer the later. Also, should the legs be as close to 90 degrees as possible when the legs are touching or at a lower angle (ie, the legs are tilted slightly away from the body?

[tyciol]

That makes me wonder, even though the hips need to flex for the pelvis to make the right angle, since some people can flex past 90 degrees, what happens to splits that way?