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 as a haishu, 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 points upward, and the back of the hand makes contact instead of the shutō. Because the lateral metacarpal receives the brunt of the impact in this strike, it is inappropriate to use haishu uchi against hard or bony targets.
[video of haishu uchi from the front and side]