The stripes are the "overlap" area. To get Boom to trigger, you have to make the pointer cross all the way to the far end of the striped area. To get the Boom to stop, you need to push it back all the way down through the striped area again.
It was put in place to create a lag, preventing a system sitting right on the border between two States and flip-flopping rapidly back and forth between the two states. That would have created an exploitable situation - such as a cmdr keeping a faction right on the brink, and nudging it back and forth, skimming profits off the top without ever having to leave spacedock.