All brakes on a trailer are connected to one wire so they all get the same current. All brakes on the tow vehicle are connected to the hydraulic system so they get the same braking.
An air brake system has valves that open and light braking may not activate all brakes causing some to not share the load. A trucker may be pulling a trailer of unknown braking ability.
Keeping this old wives tale going is only promoting the idea that playing with brakes in some fashion will get an inexperienced driver to the bottom of the mountain safely. Causing cycles of high heat then no heat repeatedly is not going to do anything worthwhile.