You are uncoupling a converter dolly from a set of doubles. Why must you never release the pintle hook while the dolly is still under the rear trailer?

Explanation

If you pop the pintle hook loose while the dolly is still tucked under the rear trailer, the tow bar can snap upward, risking injury to anyone nearby and leaving the dolly stuck in a position that's very hard to re-couple. The mechanism is that the rear trailer's weight is still bearing on the dolly's fifth wheel, so the tongue is loaded; unlatch the front connection while that load is on it and the tongue is free to spring upward. The correct order is to pull the dolly out from under the rear trailer first, and only then unhitch it from the trailer ahead. 49 CFR 383.115(a) names assembly and hookup procedures as a required knowledge area without stating this step, so it is attributed to the manual. The other options misidentify the hazard or dismiss it: no air-line rupture or permanent fifth-wheel lock is involved, and applying the dolly's spring brakes does nothing about a loaded tongue, so it does not make the shortcut safe.

Source: 49 CFR 383.115(a)