In reply to Mercedes Brugh.

It's actually NOT very intuitive. The hose is nearly full, except for the last few inches on either end. As such, you have to hold the end upright to keep the water from running out. The two ends are the only things that count; the parts in the middle can be wherever you want, coiled, straight, level, flat, doesn't matter. But the two upright ends will have the same water level regardless. That also means that if you raise one end a lot higher than the other, the water will return to level by overflowing the lower of the two ends... until they are again level (at which time, the lower end will have water right to the brim).

To use this practically, you have a central marker (in this case, the pole in the foreground) with a line on it. That line is a known distance above where you want the floor level to be. Say, three feet up. You put one end of the hose lined up with the known marker, and take the other end around to anywhere you need to set the height correctly. You then measure three feet DOWN to the place you're trying to find, and there you are! Level.