I have to go with K man on my first impression. All that hand-changing struck me immediately as something I probably (well, almost certainly) wouldn't try myself. If you control the opponent's arm/fist, don't let go of it (or at the least, maintain the block) until you move out of range. And if you have a open strike, take it as many times as you feel it's safe. I mean, if I have control of your arm and an open strike....well, shoot. Why give that up to switch and maybe lose control? It looks cool and it's a fun drill maybe, but as a practical application if that's the discussion....maybe I'm just too slow to make it work. I seem to think that one fairly simple explanation for Heian Nidan might be left hand back fist blocks incoming right hand, then same (left hand) hand does hammer fist block on incoming left hand, defender simultaneously strikes (attacker's) left elbow with roundhouse right hammer fist, then defender counter attacks with straight left hand. All fast...1,2,3. There's lots of esoteric possibilities but this is fairly simple, (sort-of) realistic set-up. Stance and body weight shifts to match techniques. I am probably stating the obvious with this, and apologize to the seniors for doing so. Edit: I found and added the video below after posting.