Synchronous Pistons is a four
ball trick in which two balls are thrown vertically at the same time,
interwoven with another pair of balls which are thrown vertically as the
first pair fall back down. As stated in its name, Synchronous Pistons is
basically just Pistons which is thrown simultaneously, instead of in a
staggered asynchronous pattern.
This short tutorial is going to assume you already know Four Ball Columns. If you don't, learning Synchronous Pistons is still fairly intuitive, and shouldn't necessitate more than simply starting with two balls in each hand and making the two pairs of throws in the proper configuration. Then, simply add in extra throws as you become more comfortable.
Now, assuming you know the Four Ball Columns, the easiest way to learn this trick is to hijack the timing of the Columns trick and simply shift the location of your throws and catches. Start by juggling Four Ball Columns, and then take your center pair of throws and slightly angle them toward a side of your body, such that one of the center balls now occupies the space that would normally be taken up by one of the outside balls. Then, shift your outside pair of balls such that one of them gets thrown in between the first pair of balls (the ones you shifted over).
After transitioning, just keep making vertical throws from the same hand position, and you will be juggling Synchronous Pistons.