When it works, it's quite impressive. The locomotive uncouples from the train, drives to the other end of the train, and re-couples. Of course there has to be a suitable path for it to do this, and it can be very slow, or not work at all, if there are unsuitable signals in the path. The command simulates standard practice on UK steam-era passenger trains.
There is also a "Run Around Train Extended" command, but I have never found out what its extended capabilities are.
Mick