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Some guy from Poland I think, maybe Lipek?

Exactly^^^

1 on 1 with aggressive defense works best, but you gotta go at it for 2 games up to 11 or so in order to really be warmed up.

I wonder if playing aggressive D, besides getting you warmed up, also improves lower ab activation for a short while, since you're staying in that hip hinge position when you defend, and you need to keep your low abs contracted well in order not to fall backwards (obviously you will let your chest down a bit as well but still)

Yeah you can also experiment with your head position.

I personally have this tendency to look at the ground when I jump off two feet and I suck so hard when I do it - I basically orient my chest down and jump forward and low.

When I actually look at the rim, which is very difficult to do for me when I jump off two feet, I get higher. Pretty much always.

skipping and karaoke warmup

People must be looking at you really weird when you start singing at the track as part of your warmup

You should experiment jumping more relaxed instead of more tensed up like you're doing right now. Right now you're thinking "I want to give my all in this jump and jump as high as possible" but instead you're tensing up and jump lower - I know that because I used to do it too and I still am doing it.

Weirdly enough, you're more likely to tense up when you're more tired than when you're fresh, so it might give you some feedback on that too - if you tense up all the time you might be overtraining. The reason for that is that when you're tired CNS-wise you're weaker and in order to "get up there" you need to recruit more muscle (or you think you need to) and you tense up. This messes up everything.

So next time try to do some more relaxed jumping, especially while planting. You can really push at the end of the jump when you take off but as you plant, "be like water, friend" to quote Bruce Lee.

PS. Shaving will give you 2 more inches.

He could try some wobble board pistols for a few months... they could help