What exactly does the white on the level border mean? I can't tell what causes it to go down and I'm only 90% sure it causes enemies to start spawning way faster.
Immediately realized that the reason I can't tell is that it takes a while for the last little bit to drain, so it's moving for a long while after I kill an enemy, which seems to be the trigger, and it seems to indicate a final wave, which happens slightly before it's done draining, because it's triggered by the last enemy dying and not the meter finishing emptying...?