It's just nibbling and pinching your nipples, the same as you'd do to a girl. No sessionette would refuse it, unless they have a no-touch policy. If they do touch you, this is a simple and inoffensive request.
Obviously woman have far more more nerve endings there, but men have some too and it can be surprisingly pleasurable.
I got to the age of 40 without realising, until a woman I had a brief fling with did it to me. It wasn't a session, but she was a big tall overweight but solid girl, and I didn't realise till we fell into bed just how strong she was.
Anyway, she began biting and sucking my nipples after pinning me down, and the pleasure built up until it was almost painful.
I tried to push her off to get some relief, but as I increased my effort I was amazed to realise I actually couldn't budge her. In the end I had to let out a loud squeal to get her to stop.
Like anything else, you probably have to be in quite an excited mood to begin with, for this type of play to have any effect