The full title is Tripcock Latch Relay - that last word is a bit of a clue....
I don’t know the full technicalities but it’s basically a relay that prevents Tripcock Mode being available and bypasses the time delay after tripping.
It dosen't have any direct effect on the Tripcock itself.
So when we arrive at a migration entry point, we switch the Master Control Switch to ATO (Automatic) or PM (Protected Manual) and the relay is supposed to switch over and take away TM (Tripcock Mode). As I said above, it also removes the time delay after tripping - so a train can still be physically tripped in the CBTC area but after resetting we can resume straight away in full speed using either ATO or PM.
When we arrive at a migration exit point we move the Master Control Switch back to TM and the relay should operate taking away all the CBTC stuff.
If the relay fails to switch as required a train maintainer can attempt to force it by plugging a box of tricks in.