 When starting out in Drive range, low gear was automatically selected and the low band applied under pressure modulated by a vacuum modulator which sensed engine output and adjusted clutch and band apply pressures to provide smooth engagement without slip.As the vehicle accelerated, rear pump output would increase and initiate a shift to direct.The rear pump pressures were modified by a throttle valve, and governor, both of which could delay the upshift, or even trigger a forced downshift at driver command.