Plain English Meaning

The faster gear for the engine's air-compressing fan, switched on at higher altitudes so the engine can keep producing full power in thinner air.

Full Definition

On a two-speed supercharged engine, the gear setting that drives the supercharger impeller at its higher speed, producing greater manifold pressure to maintain rated power at higher altitudes where the air is thinner.

Context Anchor

Seen in discussions of supercharged aircraft engines and engine power management at altitude.

Derivation

Blower' is a long-standing mechanic's nickname for a supercharger, because it blows extra air into the engine. '' refers to the higher of the two gear ratios driving the blower. So '' simply means the supercharger running in its high-speed gear.