It looks to me that the steel piece within the red square is designed to rotate around the pivot at the nearest end (nearest to us). If, from the perspective of the sitting pianist, the lever was rotated clockwise 90 degrees, the lever would be in a vertical position. The slight bend near the pivot would cause the lever to lean forward toward the pianist. The sharp, 90 degree bend at the end of the level would provide a pad that the desk coulld rest against.
My guess is that the device is designed to provide an additional resting position for the piano desk. And, it looks to me like it would be a further reduction in height from the lowest position offered by the notches in the original adjustment bar behind the center of the desk. It certainly appears to be an aftermarket device, and not original.
I do not know what advantage one would get from using this device. I wonder if the lowered position of the desk is useful for recording when the microphones are close to the strings and sounds emanating from the pianist and keys are being limited. Or, if it uses the deeply lowered desk to form somewhat of a sound trap between the underside of of the desk and the top of the plate.
I anxiously await the correct answer.