A distinctive swift of lowland forest that appears nearly tailless. The wings appear “pinched” at the body but are otherwise fairly straight and broad. Distinctively patterned, with a black throat, wings, and back, a white belly, a white vent, and a narrow white band on top of the rump. Vocalizations and nests are undescribed. Overall pattern is similar to Sabine’s Spinetail, but Cassin’s extremely short tail gives it a unique, forward-heavy profile.