Start running (W)
Make sure you are running and not walking slowly up/down a hill
Press and hold left control (if you were sprinting (shift) and have a weapon, the sprint gets canceled and you run with default run speed. Pressing or not pressing shift doesn't make a difference)
Run up/down a hill that is steep enough so that the character should walk up/down it
Observe that you will run up/down the hill with normal (non sprint) speed as long as you are still pressing control
Release control and you will start walking normally

You can then get to running speed again by running the hill sideways, press control and go up/down while running again

Video: http://youtu.be/s4lDez02jsc

Is left control hard coded as some kind of turbo key? I don't see anything special bound that key in settings or in the arma3profile file and this works for everyone by default.