Specific conditions have to be met during the match to trigger the ability to perform a brutality. These conditions include stuff like not blocking.
The triggers vary from brutality to brutality, but these are the general rules:
-So far, they are all activated by either a throw or an enhanced/meter-burned special move
-Opponent must have low enough health to be killed by the attack in which the brutality activates (because the brutality doesn't do any damage)
-Must be the winning round of the match
Also, some brutalities are variation-specific.