When you're about to start down a steep downgrade, release the gas pedal, shift into a low gear, and use engine braking to slow your vehicle. Don't try to downshift after your speed has already built up. You will not be able to shift into a lower gear then. You may not even be able to get back into any gear and all engine braking effect will be lost. If you try to downshift an automatic transmission after your speed has already built up, you may damage the transmission.