Ayden, basically what happens and this would be true of any rotary compressor, not just centrifugal, is that the compressor is creating pressure on one side of the impeller by forcing a fluid through a converging section (opposite of a nozzle). As new fluid enters the compressor the impeller "pushes" it downstream and that fluid then pushes on the stuff that entered just before it, and that fluid pushes on the fluid in front of it, etc, etc. This takes the lower pressure fluid that enters and creates higher pressure downstream. If the flow decreases to the surge line, there isn't enough fluid to keep pushing on the stuff downstream. Since the stuff downstream is at a higher pressure than the fluid entering AND there isn't enough fluid to maintain the pressure, the flow reverses and you have surge.