Don't forget about actual medical conditions that may be causing hand pump. I was following all that advice and testing different bars, grips, bar ends, bikes, etc. Got to a point where I couldn't even open a door knob w my left hand. That's when a friend who's a doctor, and mountain biker, told me to get checked out for carpal tunnel syndrome. Turned out to be severe, had surgery, and a few months later it's unbelievable how much better my hands are. Lost a considerable amount of muscle in the 7 months I wasn't able to use my bike, but no hand pump anymore!
Thank you so much for the tips regarding dropping the heels! Since I've started focusing on shifting my weight more towards the back, I found it easier to get rid of my death grip and I can finally ride a whole session without completely dying. I had the worst hand pump ever after only an hour of riding before.
I find grip size is crucial, most grips are to small for my hands which have spent most of their life swinging hammers and holding drill, shovels and joy sticks. I would say grips to small will increase the amount of white knuckle pain from clenching. Great video though, I didn’t consider the rear weight distribution for down hill