This loop relies on Selvala, Heart of the Wilds producing enough mana to sustain itself while triggering its own card-drawing and power-checking abilities on every iteration. By paying to activate Temur Sabertooth, you bounce Selvala back to your hand so she can be recast, resetting her entry trigger and keeping the mana flow positive. Meanwhile, Urabrask the Hidden ensures that Selvala immediately gains haste upon re-entering the battlefield, allowing her to be tapped for mana once again without waiting a turn.