How does a Variable Geometry Turbocharger (VGT) improve boost response at low engine speeds?

1. By using an electric motor to spin the compressor wheel until exhaust pressure builds
2. By using adjustable vanes inside the turbine housing to narrow the exhaust flow path, speeding up the gas velocity hitting the turbine wheel
3. By routing air through a second, larger compressor housing
4. By bypassing the intercooler during low-RPM acceleration