IIRC, it's based on Light, Heavy and Exploding dice, also know as Red, Blue and Black
Light/Black succeed on 4+
Heavy/Blue on 4&5, 6's count as 2 successes
Exploding/Red as is in Firestorm currently.
It was used in Planetfall & Taskforce, and was used in tandem with degradation affecting successes rolled rather than the dice pool. Whilst I don't mind the dice types, the whole successes being reduced fights against the whole exploding die mechanic.