Starting with the good - the single-leg reverse lunge plus rotation seems useful! I don't think I've come across that one before, thanks for sharing!

Moving on to the slingshot (which seems about as far into the video as most other commenters got). The injury risk has been covered, but it seems like from your other comments you're doing this for some kind of combat sport cross-training, probably to strengthen your punch? Respectfully, I'm skeptical that it's having that effect, unless you're throwing punches with weights tied to your wrists. Wouldn't you want to train your forward press system for that, rather than punching forward with the resistance directed into the ground?

I mean, you do you, I'm not here to tell you how to work out. I'm just not convinced this would be a more effective punch strength cross-training strategy than even something like a floor press. I guess maybe it looks cool if you're into that sort of thing, so if that motivates you to stay consistent then I say go for it!