It's when the turbo cuts off it causes a vacuum.
When you see boost gauges you'll often notice they aren't like the ones in LFS, but instead have a "0" somewhere in there, and positive and negative numbers, and when the turbo shuts off if you watch the gauge, it will go beyond 0 into the negative zone.
+1 from me, I've always thought something was weird about the turbo gauges in LFS.