As for the question of david nc...yes if you think about it it seems strange.but i think the answer is that you may not feeling the strap, but you know its there.you are accustomized to have / know you have something between your cheeks.so when its not there, you know it.and since the backstrap gives you mental pleasure because of its presence, when its not in your ass you are missing first if all this mental satisfaction and secondly the backstrap itself.
I think that swimmer78 can answer this question in a much better way than me....