In a fully approved procedure, an aircraft can back away from the gate under its own power under reverse thrust. However, before the jet can move backwards, it must roll forward slightly before the reversers can be deployed, and to move off of the tire’s ‘flat spot’ created when the aircraft sits parked on the ramp for any length of time.
A critical component necessary for this procedure is the ramp agent or ground marshaller. After engines are started with the jet still parked at the gate, the marshaller signals the pilot in command when to move forward, and then using a rotating motion rapidly moving the signaling wands one-over-the-other, indicates exactly when to deploy the reversers and back away from the gate. When the aircraft has safely cleared the ramp area, the reversers are closed, throttles are brought back to idle, and the airliner can then taxi out to the runway. (See What are thrust reversers?)