At the rear of the abdomen a cockroach carries a pair of cerci, each covered in hundreds of fine filiform hairs. Each hair sits in a socket that constrains it to bend along one axis, and the population of hairs across both cerci samples air movement from all directions. A hand or foot approaching at speed pushes a bow wave of air ahead of it, and that is what the animal detects.
The signal is routed through giant interneurons that run forward to the thoracic ganglia controlling the legs, bypassing much of the usual processing. Measured latencies from air puff to the start of the turn are on the order of a few tens of milliseconds, and the roach turns away from the source before running. Running speed is high for the body size, with Periplaneta americana measured at over 1.5 metres per second.
The reflex is not perfect. Its directional accuracy declines with slow-moving stimuli that produce a weak air disturbance, which is the basis of the practical advice to approach a cockroach slowly rather than quickly.