Theory to reduce oozing

[soofle616]

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.

[Mjolinor]

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.

Ideally the pressure is reduced to atmospheric at the nozzle end and held there. This will stop air entering and plastic leaving. It is a pig to set exactly right as it varies with each different plastic and lots of other things.

[3DPBuser]

Surface tension of the plastic on the walls, and mass momentum, might play a part too.

[whythelolface]

soofle, I've noticed this too in some nozzles. To remove that taper in the nozzle you can stick a flattened drill bit down there and just shave the shoulder off. An end mill might take some wall with it. You can flatten a drill bit on a bench grinder or with an angle grinder if you're careful.
Since there is already a pilot hole it works great, but you do end up ruining a drill bit.