Although they are called throttle levers they do in fact control the boost pressure via a control system that measures and holds the boost pressure at a constant level depending on the lever position.
The lower four central levers maintain the RPM at a predetermined level again by a hydraulic system controlling on the propeller pitch. Thus to drop the RPM the pitch is automatically coarsened and vice versa
Four small levers, two each either side of the centre section are the fuel cocks to the engines.