This clever loop turns Yurlok of Scorch Thrash into a lethal weapon by feeding its mana-producing ability into Hateflayer's untap-and-damage trick, which is permanently granted to Yurlok thanks to Agatha's Soul Cauldron exiling Hateflayer and granting its activated ability via a +1/+1 counter. Training Grounds is the essential engine that makes this go infinite by slashing the activation cost of that borrowed ability down to just a single red mana. Meanwhile, Yurlok of Scorch Thrash also ensures that any unspent mana left floating from this process immediately drains life away, turning your mana-pumping extravaganza into a win condition.