This piston design typically fires in perfect sync, making it potentially the best of it's kind as it is fully reversible. However, as of the latest update, it now acts uncontrollably, going backward mid-flight, braking, and utterly decimating its functionality. Prior to the 0.15.4 update, it worked beautifully. The only corrections would be in this case reversions.

This design only features sticky pistons (x2), slime blocks (x4), and observer blocks (x2) observing the air blocks to their left and right.

EDIT: Mechanism Not Included:
Piston A on left, Piston B on right
Player manually charges A, moving B to the right and giving B a short pulse.
Because A is extended, B cannot move A.
Player manually removes charge from A, bringing B to original position, gives B a pulse, and moves A to the left.
*Because A moved, it now extends, preventing the A system from moving at the end of B's pulse.
A's pulse ends and pulls B to the left, causing it to pulse and move A to the left.
It repeats from the asterisk.

As of the update, observer pulse lengths seem to be different lengths and thereby break the system.