1. Slow down. I can't believe you aren't throwing the hammer through the wall. Looks like there is a lot of effort going on in your swing. It should be fairly effortless, even on slow effortless swings I've had the hammer fly out my hand accidentally as the whip pulls it.
2. Backswing with a hammer. Swing the hammer forward before starting backswing and pendulum the hammer back vertically. I do not recommend starting from power pocket and unhinging into horizontal backswing - this is very unnatural motion to a heavy swing - also...
3. Lag the hammer head straight back behind handle/wrist. You are curling the hammer head around the handle and trying to spin it way too much, instead of leveraging it like hammer or stick and letting it spin centrifugally.
4. Rear arm. Hold a beverage and don't spill it.
I've been practicing trying to lag the hammer head straight behind the wrist, and can't seem to figure it out. Also slowed down the swing. I thought going out to a field and being able to actually let go off the hammer would help, but I was still curling the hammer. Link to hammer throw in the field: https://www.youtube.com/watch?v=3qjx2RRh2zs
I managed to throw one Buzzz out to 300ft from a standstill right after this video. Unable to recreate that again. All my midranges were going out to 260-270ft from a standstill.
Also been trying the hammer swing indoors while holding a glass of water. Still not keeping the hammer head straight behind the wrist. Somehow I've spilled only one drop of water, but watching the video back looks like I should have spilled more. Link: https://www.youtube.com/watch?v=THIrTEK1iSQ
Regarding slowing down, I have yet to feel like the hammer would just fly out of my hand. That must be a symptom of what I'm doing wrong.
Should I be thinking about keeping my arm straight to keep the hammer head in line with my wrist? That makes my arm feel tense, and I know I should feel pretty loose during all this.