You and your opponent each leave a solid trail behind you. Crash into any trail or a wall and you lose. Last one alive wins.
Steer: swipe in a direction (or use the arrow keys). Swipes register anywhere on the screen, so a stray flick off the board still turns you.
POWER-UP: hold the green button (or space) to ram. A ram lunges you two cells and smashes through trail - use it to break free, or to cut down your opponent. It drains the fuel shown in the button; let go to refill.
Outlast them. Box them in. Ram through to win.