He’s wearing a device like The Humbler only it’s made of steel. And, she’s whipping his balls with a singletail!
So you’ve probably all seen (online, anyway) the Humbler bondage device that locks a man’s balls back behind his legs when he’s in a bent-over position, and thus tends to keep him bent over until released, while exposing his stretched scrotum for particular attention.
However it wasn’t until I saw this photo on Tumblr that I realized that it could also be used to enforce a squatting position, by locking his balls in front of his ankles.
I suspect that position is not terribly stable, and he can probably re-arrange himself a number of ways. But when the slightest motion is going to tug roughly on the already-strained family jewels, the incentive is probably to stay as docilely still as possible…