*Before* takeoff it can be used to lean at altitude - refer to the POH, I believe it was over 3000ft. Also comes in handy to burn off carbon deposits from the spark plugs if the mag-check was a bit rough.
The procedure we used was to run the engine at full RPM (3200 RPM), start leaning the mixture by turning the knob counter-clockwise until you notice a slight drop in RPM, then richen mixture again slightly by turning clockwise for one (or two?) turns and engine RPM recovered.
Edit:
Just found the reference for the S300 CBi: Leaning recommended over 3750ft. No leaning in flight (Ref. POH 4-20).
Last edited by Hawkeye0001; 6th October 2013 at 22:03.