When the check switches are placed in CHECK the sync box tells the speed bias motor the prop is in a under speed condition and commands a blade angle decrease. This allows the prop to speed up and once the blade angle decreases the prop\'s ability to control RPM is lost (pitchlocked)and the fuel control\'s \"fuel topping\" overspeed protection starts.
Once pitchlocked (overspeed condition)the avalible fuel is reduced by aprox 900 PPH per 1% so if RPM is 104.5 basically 900PPH is being by-passe. The fuel control is in /on fuel topping any time the RPM is above 103%...
The charted fuel flow versus RPM chart tells you if the fuel control is bypassing the correct ammount of fuel based on the RPM. It charts specific fuel flows for a specific range of RPM, so once pitchlock is achieved the fuel control is bypassing fuel based on the RPM or in/on fuel topping.