Does what it says on the tin: Hold down aux to start sprinting. When you run out of stamina, you stop sprinting. It's a great capability to have, especially if you are frustrated by how long it takes to walk everywhere.
I recommend increasing the default sprint speed, however. Default speed is 1.3 times faster; this doesn't seem to be quite enough. I use 2.5 times and that feels more right.
If you don't have any stamina or status bar mods, you can't see how much stamina you have left. You can't tell when you're about to stop sprinting or when to start again.