To answer your question: the important point is that you do not actively do anything, you simply observe the sensations that are happening "all by themselves." Any sensation that you feel (which happens by itself) is part of the process of relaxation, so all you need to do is focus on feeling all sensations in the area.
This means that if you are deliberately trying to "stretch" the skin, this isn't helpful, but if you feel that, by itself, the skin is stretching, then this is a good thing. Letting the skin fall and feeling the weight is very, very good.
I hope that helps.
Oh IÂ´m glad I asked you. It seems I didnÂ´t do it right. As you write I was actively doing something with my muslcles.
Btw I really enjoy this practise and now it will be even easier when IÂ´m not supposed to do anything except observing. ;D