Night Slash hits the user with great power in addition to critical hit.
The Japanese name つじぎり (Testing) is probably a reference to the way in which some particularly unscrupulous samurai would test new swords in feudal Japan. This was by hiding in wait by a road (typically a crossroads) and waiting for an unsuspecting commoner (i.e. lower class, and so with far fewer rights than the samurai class) to pass by. He would then strike to kill, and in this way learn how well his new sword would cut.