When a monster is immobilized, they can't hit you. This activates not only when they hit you, but as you attack as well.
It means that at a certain % when hit or when you hit the monster, the monster will become "useless". This is like a short-term 'godmode' for your character.
The damage from the bump also counts toward your combo, which can help in some cases.
With this, and drain, it becomes really easy to keep hp maxed.
e: the damage returned is also based on your damage range, so if you think it's too low, you must be weak. :p