Top mounted stage bottle in sidemount configuration. No matter how low the pressure in the stage gets the bottom of the tank will not rise. Notice the position of the stage bottle at the very beginning and then notice how the stage bottle hangs when clipped onto the line. Clipping it and unclipping it are both easy with a little practice.
A couple of my students do their final dive for their cave training in Hole in the Wall cave located on Merritt's Mill Pond in Marianna, FL. They are using TillyTec lights and it is easy to see how large some of the rooms in the cave are.