Tension is a physical and mental strain intrinsic to conflict. When this strain becomes overwhelming, most people break - soldiers panic and break rank, adventurers lose hope and give up, the weak freeze, beasts bolt, automatons collapse.
Those that are familiar with this tension, however, those that have survived their first few brushes with it, those that face it so often that they have internalized it, know better. They don't let it overwhelm them; instead, they let it build up and when the time is right, they let it out in a powerful burst and ride it.
Tension Bursts are very potent effects that well up from a combatant's very being; it usually reveals more about them than any amount of conversation would.