Grip vid is spot on for most part, but I'm really not trying to use the wrist in that fashion of ulnar deviation either. Everything else pounds the hammer with the wrist tightening to transfer that force through it, literally like pounding a nail, but swinging through it without that blowback of the nail/wall.
I think most people misunderstand wrist extension, as literally going from flexion to extension or flicking the wrist open. What I feel as wrist extension is the wrist or thumb being as far away as possible from my center. That means my wrist is neutral, not flexion or extension because either direction takes my thumb back toward my center. That also means my wrist will be ulnar deviated some(not max) so the tip of the thumb extends as far away as possible from my center. The arm/wrist will rotate through to keep extending the thumb/disc level or nose down through.
I would say the grip is diagonal to the disc, the knuckles go diagonal to the disc plane. Same thing when you use an actual hammer grip, your knuckles will be diagonal across the handle and the index will hook the handle higher than the rest of the fingers. If your knuckles are all the same to the handle it puts the weight and wrist out of control and weak. You want to have control of the weight/head/nose all the way start to finish.
In the hammer swing, keep your weightshift 1" so your front foot doesn't need to pivot/spin out during the swing or just keep your front foot planted. You are also curling the hammer inward somehow, either too much shoulder or elbow or wrist bend happen there. Keep the wrist/thumb/forearm/handle straight inline to each other, no curl.