Hooker Sensei showed his that pin at a seminar at the ADV dojo. Have you seen the classic tanto-tori pin where uke's knife arm is bent and then the wrist ends up against the ground , compressed towards the body ?
Well, it's like that one except this time the wrist is trapped bent against uke's own armpit, and both arms get the same treatment.
it's actually pretty funny since uke ends up on their stomach, arms in chickenwing position, and the urge to flop around is hard to resist.
My experience of it is that it's more uncomfortable than painful.
don't get so uptight, Hooker Sensei is one of the most caring Aikido instructors I've ever seen/listened to /read from; pretty ironic to hear him accused of harming an uke as he's so adamant about the philosophy of Aikido.