SLA is layered, each layer a knife sweeps over it and refills and smooths the uncured resin layer. This is submerged, so drawing the parts upwards pulls in more fluid automatically so everything can be run continuously. What I don't get is that, hydraulically most of the liquid pulled in is going to come from the oxygenated layer. But I'm guessing that reacts with the resin and neutralizes quickly when it is too far from the diffusion membrane that provides the source of new oxygen.