I've always done that for upper body tickling. It immobilizes them even more, and I like them as immobile as possible for tickling. It gives them the total helpless feeling! At first, I thought you meant having sex with them as "mounting" them. 😛 I've done that too. And it was great! I could tickle their upper body and their was nothing they could do about it! But I will say, they must've enjoyed it, because the asked me to tie them up again! And one time with my ex when I had her tied down and was tickling her, I entered her and things were going good and we were getting close when I reached down and squeezed her waist and she started laughing and wiggling around so much, I swear, it felt like I grew in size! She must have felt something too, because we had a simultaneous climax! It was incredible!