The Sub Horf rotates and fires tears upon being knocked back. Sub Horfs are very prone to knockback and slide around, allowing them to fire tears for a longer duration. They always fire tears in whatever direction they face while being pushed.
When damaged but not knocked back, like using piercing tears or Brimstone, Sub Horfs will not fire tears nor rotate; they'll only make a grunting noise. Piercing shots make them extremely easy to deal with since they won't be firing back.
Likewise, high knockback can make them shoot multiple times, so fighting them is more difficult with items that increase knockback such as Soy Milk.