The Project Accelerator is the place in Project Quantum Leap, where someone leaps. Sam went into it too early, and that's why his experiment went a little... Ca-ca, as Al said. In The Leap Back Sam used the accelerator again to be replaced with Al and to save his life. He had to sacrifice his freedom but he saved Al. In A Leap For Lisa, the young Al Calavicci at the Waiting Room was catapulted back into himself, but to a time little earlier than the time Sam had leaped into him. This was done in order to fix a mistake in the past, which was impossible to fix by Sam at the time.