This sensor uses a heated wire to actually measure the weight of incoming air. The cooler the wire becomes, means more air is passing it, into the engine. Once the mass of incoming air is known, calculating fuel ratio is almost perfect. The air mass is cross referenced with engine temp in a table, again plotting where in the table temp and air mass meet provides a basic fuel ratio. Only used on MAF systems.

Pinout diagrams

Article & images courtesy of fordfuelinjection.com.