Complex sentence: When the throttle is closed to change gear or to stop accelerating, the pressurised air has nowhere to go, which causes the turbo to suddenly slow down, which causes stress on the turbines and the turbine shaft.
Split into simpler sentences: When the throttle is closed to change gear or to stop accelerating, the pressurised air has nowhere to go, which causes the turbo to suddenly slow down. This causes stress on the turbines and the turbine shaft.