You may find this hard to believe but I don't think I've ever said "Harder!" in a session. The reasons are twofold:

When I started a majority of my sessions involved a role play and in nearly all of them I was either punished for something I have done or else forced by torment to do something. In all of those saying "Harder" seemed out of character. As my sessions began to be less and less explicit role play there was still a subtle implicit role vibe going on and so saying "Harder!" still seemed out of character. (Connected with this when a Domme does "tell me how much you love this" shtick I feel that she is the one stepping out of character in the implicit role play. Not good)

Secondly saying "Harder!" is borderline "topping from the bottom" IMO.