This is easy to understand, but hard to explain without drawings or handwaving. First you must understand how a snail uses its slime for locomotion. Imagine the fully extended, relaxed animal resting on a thin pad of thixotropic mucus. It takes force to make the mucus flow, so nothing happens. Now imagine that the animal contracts the tip of its tail. Something's got to give. It must be the mucus under the tail, because there is less mucus there then under the rest of the creature's body. Right, now we have the tail that has moved forward, and the mucus above it has become somewhat liquefied. As soon as that starts happening the next bit of belly muscle in line begins to contract, and the process moves forward all the way to the front. It depends critically on both the nature of the motion and the thixotropy of the mucus. Doing it without mucus, you can mock up the process by running a wrinkle from one end of a rug to another. If you carefully watch a snail from its underside as it moves up a glass pane, you can actually see the waves of muscle going pale as they contract. Waves of paleness move smoothly from the posterior to the anterior end.
It is a means of locomotion with many disadvantages, but it has advantages as well; ask any snail on a thorn or a razor blade. However, one disadvantage is that the animal must leave a trail of mucus, which is rich in water, protein and carbohydrate. As long as resources are plentiful, this is no problem, but on dry cement in dry air for example, it is unacceptably expensive.
The solution is for the snail to go on tip-tummy. It exaggerates part of the wave motion of its ventral muscles so as to lift that part of its belly right off the ground. For the rest it continues the motion without modification. Only the ground where the belly has made contact becomes anointed. Using this technique, a snail can easily reduce its mucus consumption by a third; sometimes it manages a reduction of more than 50%. I am sure that species vary in their effectiveness, but I have not the slightest idea which is the best performer, and what degree of efficiency it attains.
