What is it ? : These 3 circuits are double pistons extenders which are efficient vertically and horizontally. It's compact, reliable and easy to build.

How it works ? : It works the same for all the 3 designs. To extend the double piston, both primary and secondary pistons are activated. To retract, the secondary piston is unpowered first, then the primary one is unpowered ; once it's done, a short pulse is sent to the secondary piston to catch the block which brings the whole system back into its initial setup.