Annular boosters on a Tunnel Ram

The reason the main air bleed starts the mains sooner (and that is a result, not necessarily the purpose of the mab) is because at different throttle openings the air moving through the carburetor is not a constant. The pressure drop and the air speed both change at different points in throttle openings and rpm. The extra air introduced into the fuel changes it’s density and that changes it’s flow characteristics.