I'd be inclined to say that I clearly see him playing with a pulling motion from the shoulder which is what is resulting in the motion that may be interpreted as a sudden relaxation of the wrist.
I dont think he is jarred or doing the drop/flop thing. I use this pull motion myself in certain situations and can say it's neither jarring nor has anything to do with a flop in the wrist. The pull action is what causesthe wrist 'flop' because it is under the pressure applied by the forearm moving down and the shoulder girdle pullin the forearm backward. It places the weight more in the wrist so it dips a little in response to increased pressure. Works well for mezzo staccato chords.
Although this is possible, something about the way his wrist rebounds suggest a pretty notable impact compared to the way he usually absorbs even the largest of drops. It's not smooth, but more of a violent recoil. You can see the hand juddering after the landing. Rubinstein was a good enough pianist, all in all, that I'm sure he could get away with this on occasion (although I really don't recall seeing this style of movement with any notable frequency in his filmed performances). However, when less experienced players try to get sound by forcing the wrist down, they get into all kinds of problems. It leaves way too much arm momentum travelling into the moment of impact. It takes a very large movement to produce even a minor level of acceleration at the finger end- and that energy does not stop itself when the key sounds. The only way to prevent that is an extremely active hand movement to absorb and redirect it. In this situation, a relaxed and inert hand makes for as much impact as a stiff one. Rubinstein lets his knuckles collapse slightly there (rather than use the fingers to move them up and away), which sends a lot of momentum into landing at the keybed.
A pianist who used that style of movement for every loud chord would soon get into problems- due to the abrupt deceleration of the wrist. There's no worse kind of crash landing than that comes from pulling the wrist down. A smooth deceleration just isn't possible.