Unlike Death Coil, Drain Life, Soul Leech, etc, Haunt's heal is taking into account Overkill damage. This means that one can use this on critters or any other one shot mobs and still receive the full benefits of the Haunt heal. Cheap fast healing.
Proof:
http://img211.imageshack.us/img211/6480/wowscrnshot031109103111.jpg
by xmagusx