You may be thinking of the ones on the Jubilee line, which I believe are decommissioned (or if not decommissioned, deliberately not advertised and sealed up). The Victoria and Central lines do have Platform emergency plungers/Emergency Stop Plungers which are advertised to the public.But as I understand it, Tube emergency stop buttons (which, as an aside, are often badly marked or even secured closed with plastic tabs, which always strikes me as odd).
The way they work is to withdraw the codes, which will cause the train to stop, or if it's already in the platform, it won't be able to depart. This is unless the train is in Restricted Manual mode, where there is no protection anyway. Not sure how it would work on the Mainline though, as whilst it could maintain a signal at the end of the platform at danger, it would have to be set up to send a message to the train somehow.