Modern Turbochargers use a VNT type design, also known as a variable geometry. The turbo shaft speed is controlled by a nozzle ring and vanes which direct air to the turbo Turbine Wheel. As turbo shaft speed reaches optimum speed, the turbo vanes open and allow exhaust gases to pass around the Turbine Wheel and not through the Turbo Blades.
If too much air is directed to the Turbine Wheel, the Turbocharger will over speed. This will pump too much air into the engine and cause running issues and possible damage. Many turbocharged vehicle's ECU can see too much air flow and will put the engine into limp mode. As engines consume less air at lesser RPM, a way of restricting the air at low RPM is required. This is done via a locked off grub screw or as it is more commonly known, a Turbo VNT Stop Screw. If the Turbo VNT Stop Screw is unscrewed too far, you will (if you are lucky) find the turbo seems to not be boosting, chances are the engine's ECU has entered limp mode as it can sense the turbo is over boosting. If you are unlucky, the ECU will not see this and you can destroy the turbo and the engine.
If the Turbo VNT Stop Screw is screwed in too far, the turbo will not begin boosting until a higher RPM. This will put excessive load on the engine, cause over fuelling, sticking vanes and can result in turbo failure and again engine damage.