Murphy runs over to let him do it, purrs, stays in that one position so he can get swooshed around, and when it's over stays there and looks like he's hoping for more.
How do you know if something might be hurtful to them? Can I assume that, based on him eagerly participating and not running away, that he likes it?
My vote is that he likes it. We do this to Muffin too. He'll be lying on the floor, staring at us and purring away (silly boy...) and one of use will go over to him, grab his back feets (that he sticks up in the air for this) and spin him around in circles. He purrs louder, and will walk over to you, flop onto the floor, and wave his back feet at you to ask for it. It's one of his favorite games.
He also loves being tossed in the air, like people do with babies. It's a game he only does with my SO. He'll go over to 'daddy', and my SO will pick him up under his armpits, and toss him into the air. His tail lashes, and he makes an alarmed face, but if his daddy puts him down after only one toss he gets agitated and runs back for more. *shrugs* He's enjoyed it since he was a kitten.
I think it's Rachel's Magneto that likes spankings??? Doran does too. If I want to play tag with him I'll pat his bum hard, so you hear a solid 'thump thump' but not hard enough to bruise or hurt, and Doran gets super excited, sticks him bum out for more, then rockets away. He does a lap around our main level, then comes back for more. I do that a few times to get him really wound up, then I'll chase him. I whap him again when I catch him, then I turn and run away and he chases me. He LOVES it.
I've said this before to, but one of the games the cats seem to really like is the 'annoy the kitty' game my SO plays with them. He'll sit near them, not looking at them until they look away, then he'll poke them and pretend he wasn't doing anything. Think like a little brother being annoying. Then the kitty stares at him, purposely looks away, and tries to catch him when he goes to poke them again. Another version is when he holds his finger close enough to their ear that he's just brushing their ear hairs. Eventually they'll get bored of this part, and then they'll play that game where they try to put their paw on top of your hand, then if you put your hand on their paw they put their paw on top, ect.
I have no idea why, but our cats do come up to my SO and ask for these games to start. Doran or Muffin will walk up to my SO and put their paw on his hand, the start of that game, or they'll walk up to him sit next to him and pointedly ignore him. It's hard to explain without seeing it, but it's obvious to me that they enjoy the games. I make a point of watching their body language, and making sure they aren't actually irritated and bothered, but they always go back for more!