The woman hugs the guys who is spreadeagled and tied. Naked body touching naked body. Rather than caress his dick she lifts her knee into his balls. She can aim for one nut or both nuts --- and know she won't miss. The sound of her knee hitting his ball(s) is a real turn on, too. Plus, it's very hard to fake --- no banging sound as a metal or plastic cup is attacked by boots or knee. Seeing the balls being hit is so much better than not.

Even more so for grabbing. She grabs his balls and squeezes. Easy when he's naked. And close ups can show whether she is really squeezing hard or not (say, white marks left by the fingers which disappear a second after she lets go).

The same for punches, slaps, kicks, even biting.

For me, it's not even close.