The thumb cylinder is plumbed in parallel with the bucket cylinder so that if there were no other valving, if you curl the bucket in, the thumb would move down. Conversely, lifting the bucket up will raise the thumb. Obviously this won't work that way as we need to be able to "lock" the thumb down at various positions so the bucket can curl or close into the thumb. This is accomplished by adding an electrically operated, normally closed, hydraulic valve. There is a button on the control stick to open that valve.
In use, if the bucket has resistance to its movement (i.e. it is touching the ground or an object), then pushing the button to open the thumb cylinder valve will cause the thumb to extend. Releasing the button will close the valve and the flow (pressure) then goes to the bucket curl cylinder to curl the bucket into the thumb. Thumb is retracted by pushing the button and "extending" the bucket - the bucket's weight gives primacy to the thumb retract so it moves first.
In use, this all comes easily and naturally. There is but one limitation I'm aware of. If you have picked up an object, rock, log etc. you cannot pivot it's position by say opening the bucket further while extending the thumb to follow and maintain grip.