The what? ???

As I was clueless on this I had to look it up.

Here is an exerpt ffrom the discussion of a different a/c (twin engine) that explains its function.....

The engine type has a reverse flow layout, air entering via a cylindrical mesh screen situated within the plenum chamber. The air intake lip-skin de-icing systems utilise electrical heating and inertial separators are fitted at the rear of the trunking. The latter systems consist of two mechanically linked vanes in each intake. When selected ON, a forward hinged flap on the upper surface within the intake trunking moves downward reducing the cross-section of the intake flow path and deflecting the airflow through a more acute angle as it enters the plenum chamber. Solids or liquids in the intake flow thus centrifuge to the outer circumference of the flow path and exit to the atmosphere via the bypass door (ie, the second vane) thus preventing clogging of the compressor inlet screen. The inertial seperator systems are controlled by two adjacent switches on the flight deck, one for each engine.

Cheers,

Dave

ENGINE INERTIAL SEPARATOR

An inertial separator system in the engine air inlet duct prevents moisture particles from entering the compressor air inlet plenum when in bypass mode. The inertial separator consists of two movable vanes and a fixed airfoil which, during normal operation, route the inlet air through a gentle turn into the compressor air inlet plenum. When separation of moisture particles is desired, the vanes are positioned so that the inlet air is forced to execute a sharp turn in order to enter the inlet plenum. This sharp turn causes any moisture particles to separate from the inlet air and discharge overboard through the inertial separator outlet in the left side of the cowling.
    The T-handle is labeled "BYPASS-PULL" and "NORMAL-PUSH." The BYPASS position should be used when flying through visible moisture such as clouds, rain, snow, and ice crystals with an outside air temperature of 4° C or less. It may also be used for ground operations or takeoffs from dusty, sandy field conditions to minimize ingestion of foreign particles into the compressor. The NORMAL position is used for all other operations.

Note: When moving the inertial separator control from the BYPASS to the NORMAL position during flight, reduction of engine power will reduce the control forces. Care should also be taken to avoid allowing the separator vanes to slam from one position to the other under the force of the airloads. Maintain a firm grip on the T-handle when operating the inertial separator.