This loop is fueled by Phantom Train, which lets you sacrifice a creature to grow larger while temporarily turning itself into a Spirit. Because of Essence of the Wild, your creatures enter the battlefield as copies of Essence of the Wild itself rather than their normal selves—crucially avoiding the Spirit subtype so they remain eligible sacrifices. Whenever one of those non-Spirit copies is fed to the train, Requiem Angel triggers to spawn a fresh 1/1 Spirit token. That token immediately transforms into another copy of Essence of the Wild upon entry, keeping your sacrificial fodder fully replenished and ready to feed the cycle infinitely.