This is a mite off topic, so please excuse my transgression eh?
Coolsti, backfire will remain for its full duration regardless of how many spells the target casts, and will punish the caster for any
completed casts of spells during that duration. This means that interrupting a backfired player is kinda counterproductive... But on the other hand, a good player will only cast if it is strictly neccessary while under backfire, it being a powerful disincentive to cast.
That backfire behaves like this means it has one of three effects:
1. A bad player will continue spamming, oblivious to the massive health loss of each spell. Many will commit suicide this way.
2. A better player might cease casting altogether, meaning a virtually complete shutdown. Yay!
3. A good player will carefully watch the situation, and gauge the risk of casting through backfire as opposed to the loss of not casting.
It is only a player of type 3 that it can be productive to interrupt even while he/she is hexed with backfire - Every cast is likely to be of great import, either an attack of opportunity (meant to kill a teammate of yours) or a defense (meant to save a teammate of your opponent).
Blah. Already to verbose. Suffice to say, that interrupting (and spending time babysitting) a backfired player is more often than not a waste of time
