Originally Posted by
Edge-
I don't know the default binds (I rebound everything to my mouse thumb buttons, may as well use this fancy like 17 button mouse since I have it!) but...
I think it operates differently for every class? I never unlocked another so can't say for sure. For white, you can use a quarter of your meter to boost your overall mana generation/speed up your cooldown on your next card draw, I forget to do it pretty often but it's a good/easy way to juice things and just let you dump even a full hand of high cost cards pretty quickly or fill a hand back up a bit faster.
Using the full meter, again this is only for white, basically seemed to buff the hell outta you, turning your primary attack into your secondary attack (significantly higher damage short range cone attack, functionally your secondary skill but without a cooldown and more powerful) and summoned a bunch of angels.
Best I can tell is you may generate some spark charge passively/in combat, but the spark items that drop give you a bigger boost to that meter. Sometimes I wish I hadn't uninstalled already so I could test this out to confirm, but eh.
Edit: Fair warning, seeing a bunch of folks complaining about scaling being busted. Apparently you see huge dropoffs in damage pretty quickly for no apparent reason. Haven't confirmed, but keep an eye out if you're playing at higher difficulties.