Another way to think about this under/over thing... if you try to print very fast (in volume terms), then you get very high pressure in the nozzle. This pushes back on the filament, causing you to get under extrusion, because the extruder motor can't push as much filament out of the hole as you are asking for. As noted before, this can amount to a 10 or 20% deficiency.
At the same time, the pressure in the head is such that in the parts where you want less extrusion, such as on the slow parts going into and out of corners, or on travel moves, the plastic oozes out anyway, causing local over-extrusions.
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Nicolinux 288
Ok, I understand it now.
Very good explanation. Another thing for the cheat sheet
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