Ok, if you place a jar, and attach some pipe, and click the jar and the pipe with the resonator you will see that the Jar has a suction of 32, and clicking along the pipe you can see that suction propagate, but pipe is one point lower than the previous.
When there is essentia in a pipe, it will always be "sucked" into an adjacent pipe with higher suction.
Now, if you place a piece of directional essentia tubing in the middle of the pipe and again use the essentia resonator to probe whats happening. In one direction, the suction of the jar will propagate normally across the directional tube. If you rotate it so that the marker stripes are on the opposite side to the suction source, you will see that the suction in the directional tube, and all the tubes further away, will now be zero.
This allows you to prevent "backflow" into a section of your pipe network where the suction is for whatever reason quite variable.