Found this bit of 90’s trucking film…like the bit with the switch for just putting on the trailer brake lights for tailgaters 
…youtube.com/watch?v=b_RyoicUrtE
Job was so much easier then, less modern tackle, but no big brother looking over your shoulder all the time and a myriad of rules n regs to follow.
Rearaxle can i suggest that both you and that driver have got things wrong - in different ways.
What you refer to as Dead Man’s Switch that you misheard only operates the brake lights is in fact an air valve lever that only operates the trailer brakes.(no brake lights) Mostly found on Scanias (as indeed the unit in the film was) and very useful it was too. If on subbie work you used the company’s trailer brake linings to slow down and your own, unit brakes, just to stop.
He calls it Dead Man’s Handle.
Dead Man’s Handle is, AFAIK, a nickname for the secondary brake - the 1st quadrant of the park brake which applies all unit & trailer brakes in the event of a failure of the foot operated system.
Unfortunately many artic drivers are blissfully unaware of its presence and function.
By solely applying the trailer brakes it could also potentially rescue a jack-knife. I think it would be worthwhile, even with all the modern safety features, having it when towing a trailer with a Moffett.
Is that when you push the handbrake in and down a little? You can feel the brakes release on either the unit or trailer?
stagedriver:
Is that when you push the handbrake in and down a little? You can feel the brakes release on either the unit or trailer?
That releases the trailer brakes and only leaves the unit brakes on.
Primarily used to assess, while parked on a gradient, if the unit brakes alone will hold the combination if the trailer loses all its air.
Not something you would need in Britain. You probably wouldn’t park on a hill steep enough.
Isn’t that what happens in the movie black dog? He flicks a switch in cab and trailer brake lights come on? Or something similar?
Yank trucks still have a separate trailer handbrake.
I was under the impression that a dead mans handle was a lever that was held down so that if something happened to the driver it would cut power and apply the brakes. This was/is used on trains as a failsafe device.
I know tube trains have that set up
Own Account Driver:
By solely applying the trailer brakes it could also potentially rescue a jack-knife.
Had a three series Scania with said trailer brake I used in icy conditions on downhill gradients kept the outfit nice and straight …the only thing twitching used to be my ar*e 
aye ,they call it a spike over here ,mainly use it for moving 5th wheel ,sliding trailer axles.
usually on steering column,always has not for parking on it.