Brainless Leg-Removers appear to have a clam-like head that is large in proportion to its body. Their depiction shows small, perhaps, non-functioning wings, and one pair of legs. They appear to be rather small, so would most likely maim a human (by biting off their leg) than killing them.
They remain under the sands motionless, but at the slightest vibration from something moving on the surface, they spring up out of the sand and snap onto its victim, presumably on the leg.
This Reference Book details the Brainless Leg-Remover and provides a pictoral view of how they hunt.