How does the special attack work?

Referring to the stat drain, does it apply only on a hit, or does it work on a 0 as well?

It should work on 0s as well, atleast it used to (haven't tested it in a while).

does not currently do anything special in addition

edit nvm found it, it doesnt do anything vs npcs but vs players it reduces all combat stats by any FLAT reduction of -0 to -15 levels randomly with any hit (including 0 hit).

Wiki explains it pretty well: has the ability to temporarily lower the opponent's combat stats by up to 15 percent.

This means the special attack can drain stats by 0-15%, rounded up (largest drain was 99 -> 84, which is 15% of 99 rounded up). I went to duel arena for a few test specs and noticed that how much you hit is completely irrelevant, and the drained stats are attack, strength, defence, ranged, prayer and magic. It seems the minimum effect is 0% drain to all of the stats (highly improbable) and the maximum is indeed 15% drain to ALL of the 6 stats.

My spec hits were:
 - 2 http://i.imgur.com/VrBhmyt.png        total drain of 34 levels
 - 2 http://i.imgur.com/I6y7Mi0.png         total drain of 48 levels
 - 24 http://i.imgur.com/TOynVj0.png      total drain of 19 levels
 - 25 http://i.imgur.com/ET7GDEJ.png    total drain of 48 levels

The spec has worked like this for years.