While uncoupling the rear trailer of a twin-trailer rig, why must you NEVER unlock the converter dolly's pintle hook while the dolly is still underneath the rear trailer?

49 CFR 383.115(a); AAMVA CDL Manual Section 7.2.2 "Uncoupling Twin Trailers," "Uncouple Converter Dolly" step, p. 7-3, Version July 2017

This is one of the few places the manual flags an outright injury hazard by name, which makes it a near-guaranteed candidate for test questions - and more importantly, it's a real rule that protects you physically during everyday uncoupling work.