The ability's effect is simple, when a creature that has rage is fighting (e.g. Fire Dragon), it gains attack power over time, but when it stops fighting for a certain amount of time (5 seconds to 10 seconds) then it goes back to its original damage output.
Every five seconds the unit receives a +100% base damage multiplier up to a total of +400%.
For example, a Fire Dragon will do the following damage will each successive attack:
Cards with Rage