How do you control the car?
Press your finger behind the car and while it is pressed down, sweep it left and right to steer.
How do you use RAM?
With Ram equipped in the gadget slot, start a run, and when the cop cars move along side or in front of you, an orange pip will appear showing that they are in range to be rammed. Take your finger off the screen and tap the circle that appears over the rammable cops. Once tapped, the circle turns green. Press your finger back behind your car to drive and your car will ram the cop.