Beautiful isn't it? It is a combination of the nature of the mucus under the slug's foot, and the way the muscular contractions distort it. The mucus is thixotropic, meaning that if it is allowed to settle for long enough without moving, it sets into a comparatively firm mass. Think of the slug at rest. Under its belly the mucus forms a firm layer that glues the animal to the surface.

Now imagine that a bar of muscle across the underside of the tail of the slug contracts. What will happen? The mucus under the tail cannot resist such a powerful distortion, so it liquefies. Most of the solid mucus under the slug is towards the front, with only a small patch under the tail, so which part do you suppose will give way and liquefy? My computer says it will be the part under the tail, so the tip of the tail will move forward.

Very well, so the slug's tail has made progress, and the bar of muscle across the underside of the tail can now relax. But the rest of the slug is still stuck down. Do not despair. Just in front of the bar under the tail where the contraction had taken place, the next row of muscle fibres smoothly takes up the slack, and moves the next bit which is over the liquefied part, so progressively every bit of the slug moves forward. At this point it is clear that the whole animal has moved forward by the width of the contracting bar of muscle. Clever of course, but that sounds rather unexciting even for a slug. And in fact it is too slow for practical purposes. What the slug does in practice, is that once the first wave of contraction has moved far enough forward for the mucus under the tip of the tail to have set again, it starts a new wave following the previous one. That sequence of forward moving waves is the movement that you see under the slug's belly. In this way the slug can multiply its speed by the number of waves of contraction that it can pass forward at a time.

In many ways this is a marvellous way of moving, quite apart from its sheer aesthetic loveliness. For one example it enables slugs and snails to move across razor blade edges without being injured. It also permits startling versatility of motion. For example, have you ever seen a slug tiptoe? All right, I admit I have never seen a slug's toes, but perhaps you would prefer to say that it can walk on tip tummy. When moving across a dry, absorbent surface such as a concrete slab, you will find that a snail leaves a track, not of continuous mucus, but of dabs that take only about half the amount of material. The animal can then move twice as far for the same expenditure of water and protein.  It does this by lifting parts of its undersurface from the surface in another series of waves. (And in which direction do you suppose those waves would pass?  ;-)

I have never seen a slug or snail move in reverse, but there is no reason in principle why waves of contraction in the other direction could not move the animal backwards. Once again of course, the contractions would move in the direction opposite to that of the transport of the animal. Possibly tactful stimulation of a slug in a glass tube would demonstrate this in practice. Let me know if anyone has done this or would like to try it. If so I would love to see a video, but I warn anyone concerned, that I would watch that video with a jaundiced eye, alert for any evidence of reversed playback! Another interesting experiment would be to measure the absolute and relative velocities of the motion of the animal and the motion of the waves of contraction, whether normal or "tip tummy". Small animals can suggest simple, but non-trivial lines of investigation of large interest.

Incidentally the first time I noticed this mechanism was when as a student I happened to notice smallish aquatic snails crawling on the underside of the surface of an indoor pond of still, clear water. Though they did not achieve the blistering velocities of say, slugs on a solid surface, they managed fairly well. Unlike the typical garden snail, instead of showing two or three waves of inconspicuous, mild muscular contraction passing beneath at any one time, these little fellows displayed several, rapidly moving, clearly visible, white, intense contractions. Whether they were using a sort of jet propulsion, moving water backwards to propel themselves forwards, or more likely, making use of an invisible layer of mucus on the water surface, or something similar, I do not know.

it was an eerily beautiful memory all the same.

Cheers,

Jon