Nothing, the weights will slam into end of slot and sometimes bounce back and slam the stop again eventually stabilizing in a street car at certain rpm ranges. It is an old drag racer’s mod to get the fastest advance, and where a drag car has two throttle positions; off and wide open it works well.
In a street machine the big long loop spring cushions governor weight as it swings out, and slowly allows the last three degrees of advance to creep in by about 2800 rpm. The designers of the distributor and timing curve put that spring in there for a reason.