What it does
The form at the foot of every page takes an email address and keeps it. That is the whole of what happens: the address goes on a list, and the list is real and stored.
What it does not do
Nothing reaches you. There is no mailer in this project and nothing in it that could send one, so no fare is watched on your behalf and no note goes out when a price moves.
The form used to say otherwise, in three places at once: a heading about your inbox, a line about being told when a fare dropped, and a consent line saying the address was kept in order to do it. All three were promises the site had no way to keep, and they were removed rather than made true.
Then why is it here
Because keeping the address is the honest half of the feature, and the half that can be built without pretending. If a mailer ever arrives, the list is already there and already has people on it.
It is also worth knowing what a form like this does with what you type, which is the point of saying so plainly rather than leaving you to assume the usual.
Joining twice
The list holds each address once. Give one that is already on it and you are told so, rather than being quietly added again — the same answer, whichever it was, so nothing about the reply reveals who else is on the list.
Getting off it
There is nothing to unsubscribe from, since nothing is sent. What is stored is the address itself, and Privacy covers what this site keeps and why.