He was no doubt slipping to slow down- that's what it's all about; slowing up to increase the descent rate, instead of diving, which will produce too much airspeed, or mushing down, which blocks your view of the runway, especially in something like that Yak, with its big round engine and forward-mounted wing.
Which is what puzzles me... I have no idea how he could see much at all with the forward wing raised... but I'm sure if I asked him, he'd say something like "Well, I was assuming it was still where I saw it a moment before...:

You missed my point. A conventional sideslip is used to lose altitude quickly without increasing airspeed. In this case he only needed to slow down as he was already at the correct altitude. I suppose it would help to know the layout of this place. This runway has an offset approach & he has to turn through 50 degrees to line up with it. He exaggerated the turn so the nose was pointing at 90 degrees to the runway with the aircraft almost stationary. It was only
slipping skidding sideways at this point. Then he turned in the opposite direction to line up, rather like a letter S, before gently touching down almost vertically & tailwheel first. It's a power manoeuvre which is only possible with this type of aircraft. I'll print these shots off & give them to the pilot next time I see him. I'll ask him exactly how it's done & the correct name for it.
PS. If I was still flying my R/C models I would try this manoeuvre myself. It's probably not difficult but looks very effective.