The Sub Horf rotates and fires tears upon being knocked back. Sub Horfs also seem to be more prone to knockback than regular Horfs and slide much further because so, allowing them to fire tears for a longer duration. They all ways fire tears in whatever direction they face while being pushed.
Notes
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.