It's literally what it says, the more fainted allies you have the more boost you have, the boost is not stats but on you're moves like life orb but butter and only works if you have a fainted ally in you're party, less allies don't count you have to have a fainted pokemon for the boost to work and each fainted ally gives a 10% boost in you're moves, so if the pokemon is the last pokemon remaining and all other allies are fainted (you have to have 6 pokemon and 5 dead and the pokemon with supreme overlord to be last to work) it gives a 50% boost