Exactly, it causes NOTHING to happen, so you can fully customize behavior of the skills. Though I recently discovered that it doesn't get called in some cases, like when lockpicking a door.@phobos2077, what exactly does the return value for hs_useskill do? I tried all kinds of different values for doctor when crippled and first aid when healing but all changing it from -1 does is cause nothing at all to happen.
Also, try using register_hook_proc, it really improves modularity.