It's nonlethal warning, can be executed from 'high guard' position or a 'x-guard' half-position, perhaps more.
It's similar to standard hand-slap at cheek.
With knife held in right hand, it can be executed from left to right (uchi, as in uchi-uke block, or backhand slap with a Knife) or right to left (soto, or a forehand slap).
I've had insight that resulted in discovery of this method... for both unarmed & armed fight.
Discovery is not enough, should be practiced to be an effective nonlethal warning.
Sheathed Knife can also be used to slap nonlethally.
See this film, for an example. This example includes combinations of punches with slaps or other nonlethal moves.