Mason & Hamlin Hickman Grand #40730
||Mason & Hamlin
Clarence Hickman invented a new type action for grand pianos. It was only installed in a small number of pianos before the Depression put a halt to the invention.
A very helpful website showing its operation can be seen here
In brief, the action works a little like when someone knocks your knees out from under you. In the piano, the hammer is thusly prevented from blocking against the strings. Similar to 'escapement' in the conventional grand action.
These actions also have a lever to adjust the down weight one feels at the key. Each key has a flat spring screwed to its underside. The springs bear against a rail which when adjusted controls the amount of 'spring' imparted to the key.