https://en.wikipedia.org/wiki/Driver's_reminder_appliance It's basically meant to be used as a reminder (and lockout) that the next signal is red if you've stopped and can't see the signal (or even if you can)
Also put your reverser into neutral as well. That's what I was told when I had a driving experience on the Great Central Railway
Then when you get a green signal and you hit the throttle and cant understand why you are not going you suddenly remember you have the DRA on!!! Many times that has done me. Hentis
IRL you use it or get tea and biscuits... In the sim, given we are often given the "right away" against a red signal, a more serious rule breach, it's up to the player's sense of authenticity. As I spend a high proportion of my runs with AWS, PZB, LZB etc. switched off anyway I tend not to bother.
You use the DRA whenever you stop and the next aspect is red. So if you stop at a red you put it on, also if you stop after passing a single yellow you put it on. Also in the real world you put it on when leaving a cab. But it's usually a passenger train thing, freight trains don't normally have it. And as mentioned above, quite a few TOC driving policies also require the driver to put the DDS into neutral and the brake into Full Service when you're applying the DRA. Checkout the Wiki page on DRA.
