Turbo speed limitation
WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
Courtesy of SAAB-SCANIA OF AMERICA, INC.
The maximum permissible air mass/combustion is limited at low atmospheric pressures to protect the turbo from overrevving.
The function depends on the atmospheric pressure and engine speed.