River takes a second to stare blankly at him. Why is that relevant, Duo?

Then she's following after him, vaulting the railing on a slightly different trajectory that will carry her to the floor a few yards away. (Not too very far below, and there's a thug to cushion her landing against his will too.)

Canda, they call it in the High Speech: the distance that ensures that two gunslingers can't be taken out by a single shot.