this one is always funny, anyone know why my orange would pull his ass so high to get a nice smack? almost as if they feel some sort of relief

It might just be because he loves you. Of course, it could also be:

https://en.wikipedia.org/wiki/Scent_rubbing#Domestic_cat

pretty sure its a sex thing - makes sense for females to lift up their rear and I guess there isn't any evolutionary pressure for that sensitivity to not exist in males as well.

This would be a plausible explanation tho I've been told that cats don't move their tail that way when they're in heat. It doesn't go up but rather sideways.

So maybe it's that but maybe it's something else.

Ever have someone bang on your lower back that way? It feels good :)

This is why people go to chiropractors.