What is the chance a dodge will trigger Backlash?
When using the Backlash rune for Mantra of Evasion, what does "a chance" mean?
Successfully dodging an attack has a chance to create a burst of flame dealing X% weapon damage as Fire to all nearby enemies.
No one (outside of Blizzard) knows currently. There are other abilities that simply say "a chance to..." for other classes too.
Hopefully when the Diablo 3 official game guide on the battle.net site gets updated from its current state of "basic" to some sort of "advanced", questions like this will be answered.