Certain attacks have a chance of inflicting a burst, usually only on a critical hit (see ‘help critical’). A burst is treated as an exact duplicate of the fighting technique that caused it, except that it usually inflicts a different damage type, sometimes hits a different body part, occurs instantly and never misses (although a roll to hit is still made to determine the effectiveness of the burst attack). This means that the burst has the same attack (and attack strength), damage, natural bypass, etc, as the technique that caused it.
You cannot inflict more than one burst per attack. If you have a percentage chance of causing multiple bursts of different damage types, then the mud will go through them in the following order: Storm Talons, poison, fire, lightning, sonic, frost. If a burst fails its percentage chance to perform, then the next will be checked, until a burst is performed or there are no more left to try.
Bursts of the same damage type from multiple sources do not stack. For example a vampire with Umbrage hand runes (gives a percentage chance of causing a cold burst) who has House DarkBlade and wields a darkblade (also gives a percentage chance of causing a cold burst) will only get the percentage burst chance from the darkblade, not from the runes, because both have the same damage type. If the vampire instead had the Ember rune (fire burst) then the mud would check for the fire burst first (fire has a higher priority than frost), and if that percentage failed, then check for the frost burst from the darkblade.