How to use corrupted spells?

Every corrupted area yields a corrupted spell. But I can't use them. Why? Should I de-corrupted them? How so?

It doesn't make any sense.

Vaal Skill Gem

Vaal Skills may be used only when enough souls have been collected from slain enemies. Collected souls are saved to the zone instance they were gathered in. Changing zones or resetting the instance will start a new soul counter, but returning to a prior instance will "remember" the souls you left with. Collected souls are lost if the Vaal Skill Gem is removed from its socket.

I tested it all, everything is accurate. I never saw anything about putting the vaal gem in main weapon or something. You can put it anywhere you want.

(in your gear I mean, you pervert)

Corrupted gems have to be "charged", to charge a gem, keep killing monsters with it equipped (not in your secondary weapon slot though).

They work like potions, in that you can use them after killing enough monsters to recharge them.