The “magic” of these pixels is that they're digitally controlled…even though there are only two control lines, you can have as many pixels as you’d like in a single long strand, yet each remains independently controllable.
Though it looks like the 4-conductor ribbon cable is continuous, it isn't! The pixels have a distinct “in” and “out” side. Data from the microcontroller arrives on the input side, where it’s received by the driver chip. The output side then connects to the input of the next pixel, all the way down the line.
The way to tell input from output is different among the shape and version of the pixels you’re using:
Luckily, we have them as a pair so no matter what pixels you end up with, you'll have one for input and one for output