Mmmm ....
Hawkeye is spot on WRT 'leaning' ..... engineers will do a similar thing with the injection flow control during maintenance checks to get a smooth engine idle.
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Why has it got that notched knob? ....
Because there are other 'vernier' type controls (also with different colour knobs ie: Blue or Black) which (in aeroplanes) are used to control other engine functions like 'Propellor Speed' setting or 'Throttle Position' and they also have different 'notches' or shaped so as to allow 'tactile' identification with out having to actually look at the control to know you have selected the correct one.
You have actually asked some very good questions ... well done.
