Haishu uchi (literally: “backhand strike”) is a backhanded slap to an opponent’s face, typically used as an atemi. Within Goshin-Jutsu, backhand strikes are always referred to by their Japanese name, to avoid confusion with the more common backfist strike.
The mechanics of haishu uchi are just like those of a makkikomi-shutō uchi, except the hand is rotated 90°, so the thumb could point upward, and the back of the hand makes contact instead of the shutō. Because of the lateral impacts which the numerous small bones inside the hand will experience in this strike, it is not appropriate to use haishu uchi for crushing blows to hard bony areas. Instead, make a distracting slap to soft tissues.
[video of haishu uchi from the front and side]