You ever play games with guns that would 'overheat'? I think some of the weapons in Halo are good examples of this. Each shot fired would increase the 'heat', and once it reached maximum you would have a longer period of inactivity as you cooled down (but it would also cool down a bit between shots). So you could either fire really quickly and then have an agonisingly long cool down time, or fire more slowly and prevent the weapon from reaching the overheat point.

I expect the system we're looking at here is similar to that. Every action will have some form of cost - simple attacks may only fill 1/3rd of the bar per strike but a magic attack may use the entire thing - and if you fill the bar you 'overheat' and have an extended period of inactivity. You could avoid this by attacking slower, but since you seem to have powerful finishers at the end of a combo it's more likely that you'll build up the bar and switch to a new character as you cool down.