Not familiar with that particular term but I'd bet what they're doing is closing the brake pipe angle-cock before the train-line breaks when uncoupling. Doing that causes brake pipe pressure to be maintained instead of being released thus setting the train brakes in emergency. Once Power has run around the train and re-coupled brakes can be quickly released by raising train-line pressure. If train-line pressure had been released a full recharge (filling of each car's main reservoir) would take a lot more time.
On Edit: Heh. Not a real railroader but HOTWATER is 
