Didn't watch all of the the Mori video, but the Nikkajo he demonstrates within the first 3 minutes of that video is different than the one in the video used as reference in the OP. In the OP video, the shite's hands are close together relative to most versions of this type of wrist lock, and can readily work together in a manner that allows the degree of pain from the lock to be varied (i.e. the hand not applying the lock can manipulate the uke's wrist and forearm like the tsuka of a sword, supported by vs. supporting the hand applying the lock, if the shite so chooses). Here's the close-up shown in the video: http://youtu.be/QchlmrPnidA?t=1m30s
The shite also induces kuzushi by stepping forward to drive the forearm forward and down toward the uke's center as the lock's applied (again, the force can be transferred to the uke's arm primarily via the hand grabbing the wrist moreso than the hand applying the lock, if so desired).
These factors should allow the shite to execute the technique with little or no pain, if so desired, for demonstration purposes. And, based on my experiences taking ukemi for Yoshinkan practitioners, that is the case.