What does "handrail hang-up" refer to in the AAMVA CDL Manual, and what must the driver do about it?

AAMVA CDL Manual, Section 10.2.5 – Special Dangers of Loading and Unloading, p. 10-5 (July 2017 revision); 49 CFR §383.123(a)(1)

A loose drawstring, backpack strap, or shoelace catching in a handrail or door is a real, documented cause of serious student injury and death during exit, not a hypothetical risk. Because this kind of entanglement can happen quickly and quietly, the manual puts the responsibility on the driver to actively watch each student clear the exit safely, rather than assuming everyone got off fine just because the door closed.