It's very easy. The feet are staying flat on the ground whilst you lower your hip (keeping your back straight), thus causing your knees to bend. You try it - I bet you can do it. Incidentally, since you think that the first pic looks very natural, does that extent to having feet inside the ground? But criticising my Poser skills beside the point. The point being that PoseMagic's results can be achieved via simple use of IK.