Quote Originally Posted by Mjolinor View Post
It creates a reduction in pressure.
A reduction in pressure from positive but not below ambient. If the implication is that negative pressure is drawing melted plastic out of the nozzle that makes no sense. For negative pressure to exist the filament would either have to be retracted faster than air can enter into the nozzle (theoretically possible but incredibly unlikely) or the nozzle itself would have to be clogged or otherwise impeded to prevent air from being drawn in as the filament is retracted.