Thaumcraft's essentia transport system. Its all based off the concept of suction, different things draw essentia at different rates and have a certain amount of draw to them, like any other fluid system. HOWEVER, the system is completely, utterly fucking retarded. If it can't send any essentia to one destination, it'll just let the source get backed up instead of going to another applicable destination literally ONE block further down the line, like an overflow system. It doesn't help that the machine you use break items down into their component essentia (like turning coal powder into ignis, potentia, and perditio essentia) is completely fucking stupid, as well, and won't refine other essentia from the "slurry" it stores if one buffer for one kind of essentia its attached to is full. It just stops effing working, causing the entire system to gum up and stop, which when combined with the completely idiotic transport system, makes automating the creation of certain things a far, FAR more massive undertaking than it should be.