The supercharging actually is working at low RPM and we all know that most of the Turbo will not get good result at low RPM or we call it turbo lag, so VW thinking to get good power from low to high rpm, so they mixed the supercharge and turbocharge together, low rpm the supercharge will kick in to give full torque until certain rpm, the supercharge will stop working and turbo charge will then start working until redline.