Thanks for the nice video. Up and Down looks very clearly. This kind of movements are also seen in other arts too and very universal. I think this is the secret atemi that many shihans use. IMHO, the question is how to do it without bending or un-bedning knees.
Knees are not a source of power, so just keep knees soft and aligned and don't put your mind in your knees at all.
Seriously, even when simply mechanically going down to the mat, the best single piece of advice I ever got about sparing my bum knee was from the guy in my dojo who has done years of Chinese internal arts: "don't even think about your knees, just soften and open your hips" and damn if I didn't lower all the way to the mat effortlessly. Teach it to my beginners in Low Impact now.
But in terms of the video and your question, it isn't a matter of mechanical knee stuff at all, it's internal organization and structure.