! serves as a toggle.
The ability will be cast once, assuming enough Runic Power when you push it.

Without the ! some abilities used to toggle off.
Not sure that you need the ! any longer, as I know you do not for Maul with a Feral druid.



I use (or something very similar):
#show
/cast [nomounted] !Concentration Aura; !Crusader Aura

No matter how that key is spammed each ability will only fire off once.