Activate only when an "Infernity" monster is selected as an attack target while you have no cards in your hand. Destroy the attacking monster and Special Summon 1 "Infernity" monster from your Graveyard.
Here are some of the monsters you can protect and revive with Infernity force. Infernity decks like to play with 0 cards on hand, making Infernity Force easy to activate. Revive Infernity Necromancer for further swarming.