It's the two handed diagonal fingertip strike that's performed to the rear in Seisan. In the bunkai application, it's the move that's used to grab and turn the opponent to the right and, in the bunkai applications that I've seen, into the sword strike presumably of his partner.
I believe the Japanese terminology for the technique is "morote gedan nuki." Is there a preferred target? I've seen it applied anywhere from the hips to just below the rib cage.
The technique came with a legendary explanation that the old masters had such strong fingers and thrusts that they were actually grabbing entrails

What I was taught was that the double strike dented or punched in the abdomanal area just below the ribs. You could then grab the flesh with the lower three fingers of each hand while at the same time making a shoken fist with the thumb and first finger. With this massive pinch in place, one next twisted both wrists in an outward, upward direction pulling your arms into a sanchin posture "morote" shoken kamae." The resultant pain from the twisting, pinching grab raised the opponent to his toes and thence he could be turned when you pivoted and used as your shield.
Are there other "understandings" or applications? The same technique appears in Seichin but the turn there is to the left. I've seen a Seichin bunkai but without the sword so I guess that the opponent is just used as a body between youself and an attacker.
In the Seisan bunkai I've seen all sorts of belt and gi grabs to move the opponent, so is there a trend away from what I was initially taught? These alternate grabs seem to get away from the pain and damage that the grab and pinch manoever can inflict, i. e. they are more theatrical than a truly punishing technique.
How is this technique taught and practiced since you won't have many friends if you really do it correctly?
