The spell created stone spikes to transfigure/shoot from the walls and floor, blocking the Order. One of the spikes had impaled through the portrait of Sirius's mother Walburga. And only Mundungus Fletcher got caught by a stray stone spike that impaled through his foot inciting screams of pain. But the teenagers bolted, blasting the door off the hinges and escaping the building. Harry and Hermione both knew the spell would keep their would-be captives from catching up. They would already be gone by the time that hallway of spikes was cleared away with a couple of blasting curses. Hours later, they got their bike from the park after traveling across London, and were already fleeing the city in the air.