Solo is most likely changing a fuel mixture and then restabilizing.
Flooding the intake, would analogous to a jet engine cause a change in the 'air-fuel'1 mixture of the sublight drive.
Suddenly changing the 'air-fuel' ratio of the Millenium Falcon, would, like in modern jet engines cause some instabilities and potentially stall the engine. One can assume that sweeping the bi-lats (potentially Bilateral Fuel drive stabilizers?) would compensate for this change, and allow for the sudden burst of speed.
The linked articles implies a low pressure behind the exhaust part of the drive could cause a stall, which would not be what you want. Equally, unstable 'combustion' (for lack of a better word) could also cause resonances in the engine and damage it beyond use (potentially what Needles did)
1. Or the spacefaring equivalent