Who knows?
Some drivers do things different.
The trouble with using the trolley to apply the trailer brakes is that sometimes the "friction lock" on the trolley valve isn't strong enough to hold the lever in the applied position, and the trolley can unexpectedly "pop off" while unattended.
A loss of air pressure could cause the trailer brakes to not hold properly, up until the point when the air pressure gets low enough to make the red & yellow knobs pop out & apply the spring brakes.
I always just pull the yellow knob if I'm getting out of a truck.
On most trucks nowadays with the combination dashboard valves, pulling the yellow (tractor) knob also pops the red (trailer) valve at the same time.
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