The Accelerator Pump is a 2-Stage pump that automatically switches between the stages during inflation. The switch is quite distinct because the pump changes to a different internal motor as a normal part of its operation. This typically happens somewhere between 1.0 – 2.0psi. During normal operation, after providing power to the pump and setting the pump to your desired working pressure, a brief touch of the power button will begin the pump motor in stage 1. The pump's 'volume' stage motor (stage 1) makes a high-pitched whine that sounds a bit like a vacuum cleaner. In stage 1 the pump will push as much air as possible until the air pressure gets to about 1.2 psi. Above this point, the pump will automatically change to its ‘pressure’ stage motor (stage 2), which sounds more like a small combustion engine with a cylinder. The pump determines it’s time to make the switch to stage 2 when it’s making no progress at all in stage 1. This process may take a minute or so while progress is slowing. Although the stage 2 motor moves less air, it does so more powerfully, so it has the ability to reach pressures up to 20.0psi. Inflation progress will gradually slow as the pump reaches the upper limits of its range. If you are standing nearby as the stage 1 progress begins to slow (~1.2 – 1.5 psi), you can save some time by forcing the pump into stage 2 (pressure) mode. Do this at any time during inflation by holding the power button down again for 3 seconds. Unless already operating in stage 2 mode, you should hear a distinct change in the sound of the pump as it changes over to stage 2.