Neck sealing with reduced loads. The neck of the case expands and forms a seal, keeping the burning powder in the barrel and not back flashing into the chamber.
Something to look at when you are working up low power loads.
Easy to check, polish the outside of of a case neck, shoot it and see if you can see powder burns rearwards down the neck and/or on the case.
Most of what I was doing was experimenting on cast lead bullets (for the zombie apocalypse). Lower velocities really reduces lead fouling.
If you go too low on velocity the neck may not seal and the back flash can get to be an issue.
Burning powder bypassing the neck and getting into the chamber can have long term corrosive effects on the chamber wall.
I'm sure some of the hand load experts can explain this better, maybe use better words.