Mark Bellis is one of the few gurus in LEGO pneumatics whose work I've crossed with (not to mention any other areas of technical knowledge).
He has some fascinating work done on LEGO pneumatic building blocks that simulate logical and other functions.
Recently and in the sequence of some discussions about linear actuators an solenoids, Mark that is a strong defender of the pneumatics, made a prototype for what he called "Continuously Variable Pneumatic Control". One block that allows a set of pneumatic cylinders to stop anywhere within their range, allowing pressure and force in either direction to be modulated. A pneumatic linear actuator...
Since it is not a trivial idea, neither easy to put in practice or to understand, he also decided to make a video tutorial explaining how it works.
"A revolutionary development for LEGO pneumatics, opening the door to proportional control and providing a way for LEGO pneumatics to compete with the Linear Actuator found in recent sets. " as he own wrote in the respective video description at YouTube.
You may find some photos and detailed description about this block at Mark's BS folder, or even a lot of his other ideas for pneumatics in the folders one level above.