Its basically the same thing Ando Sensei's uke was doing, except as soon as his shoulders hit he immediately pops his hips out, the presure release comes into play as you see his legs kick out...from what I can tell at least.
Thanks for the comment Adam.
I don't think Ando teaches this method as part of his kihon though I could be wrong. In the second video clip from the OP, Ando's uke clearly drops his hips and makes contact with his butt on the matt first. Basically a standard koho ukemi, with the addition of the legs kicking up at the end to relieve pressure from the pin.