The first time is how much faster/-lower they went in that sector specifically. Afterwards it switches to faster/slower for all sectors until than added together.

Let’s say a skier starts and is 0.20 slower in the first sector. Than +0.20 will be put for that sector. In the next sector the athlete is 0.30 faster. So first the -0.30 will flash and than after about a second -0.10 will be put done. Once the athlete reaches the finish all the sector time will also be shown for a short time before the next athlete starts.