Plain English Meaning

It is the slower setting on a supercharger that has two speeds. At lower altitudes the engine doesn't need as much help, so the supercharger spins its impeller at the lower ratio. As the airplane climbs higher and the air thins out, the pilot switches to the faster setting to keep the engine making full power.

Full Definition

The lower of two impeller speed settings on a two-speed supercharger, used at lower altitudes to provide moderate manifold pressure boost without overcompressing the induction air. The supercharger's gear drive turns the impeller at a reduced ratio, supplying enough additional air to rated power up to a designed before the pilot must shift to .