Hydraulic fail-safe.
Some of those old guys were pretty bright....
I can’t believe that I just figured out your handle after seeing it since 2004. Until this moment, I always read it as nas carnation. Duh!
I used to make a little money when I was in High School (’58-’62) by modifying the valve bodies of Iron Torqueflights for people that towed trailers, so that they could punch the trans into first gear for engine braking at any speed. Normally it wouldn’t do it until you were below 20 MPH.
It made towing in the mountains way safer to be able to outwit the trans.