"Not bad, looks good so far. See?, I told you I would be nice to you, didn't I slave?"
"Yes mistress, thank you mistress"
"What about your other hand though?. It should have smething to do as well, shouldn't it?. I mean, I don't know you of course. Maybe you want to stick it up your ass while you stroke?. Maybe you want to slap yourself across the face?. Slap your balls?". I'll let you chose slave"
"May I please slap my balls mistress?"
"Of course you can, go ahead"
(You slap your balls while stroking your cock)