He tries to get her to say "you luv black cock" but she doesn't comply... so he thrusts even harder which leads to her having orgasmic spasms in a retard-like manner.
Apparently getting your dick hard for a hot pornstar isn't as easy as you'd think. I'd probably blow my load before even getting my cock out of my pants.
I like to believe this has less to do with his cock and more to do with her vagina. Some girls are just extremely sensitive down there, and I think that even a 3 inch pecker such as my own could produce the same results. Don't pity me for having hope.