The shaft on which the flap handle is mounted is the piston of the master cylinder. The end of the barrow can be seen just below the camera mount. This drives the hydraulic cylinder used to actuate the flaps.

This cylinder varies the flaps from -10 degrees to about +70. The pictures are from HP-16T N16VP. The system is a "closed" with no reservoir. It was installed by the original builder and works very well.