In short, sloshing, though in detail a bit more convoluted.
Honestly I wish fluid pipes in GT6 worked like Item pipes did, a pipe could even cache the endpoints it has to distribute between and if a side fluid inventory changes on any pipe then it can replicate down the entire pipe ‘network’ to clear the cache so it gets rebuilt as the network changes so it can rebuild the cache on the next fluid insertion call. Would make fluids a lot faster on the CPU too and probably use less memory overall as well, but at the cost of you wouldn’t be able to easily detect how much fluid is flowing ‘through’ a pipe unless it is the entrance point unless that was recorded in each as well, which is just a lot of needless cost.