When you press the button on the side of the shift knob, the transmission is prevented from shifting into overdrive (or the highest gear, or whatever you want to call it). You'll also see a green arrow light up on the dash when this is toggled. Once the engine is shut off, it reverts to normal. The owner's manual explains that this is for trailer towing, but I expect they mean a house trailer or big boat trailer, that would also require trailer brakes.