At 7 minutes 36 seconds in the video below is a demonstration of a London Underground short circuiting device (from circa 1990) that connects the positive and negative current rails directly to one another - again after the current has been turned off.
The video goes into more detail about them from 8:38
As you say, such devices are to be used after the current has been shut off - and not before. They are usually employed when possessions are taken as they ensure the current rails cannot then accidentally be turned on again (any attempt to do so will immediately blow the circuit breakers again) until the work is completed.
NR also use them - however because of the use of the running rails to return the traction current on the national network, the consequences of a false recharge on the signalling equipment need to be considered - and as such there are certain locations where such bars must not be used - because any attempt to recharge will result in destroyed insulated rail joints / point tie bars / etc.