Originally posted by MrStefens
I think the thrill is in when she is not physically compelled, but something requires her psychologically to continue...her own sexual desires buried deep, mind control, the personal chemistry between her and the Dom, circumstances.
I agree with the "not physically compelled" part of your post but I personnally prefer it when the sub accepts to do 'dirty' things or to receive pain for the sole pleasure of the Top.

I love it when the sub is active in her/his own humiliation/degradation.

Cheers