This effect can be understood by considering what happens to the cantilever and tip as the base of the vibrating cantilever is brought closer to the sample (like in a force curve). When the vibrating cantilever first approaches the surface, the tip senses attractive forces. In standard imaging conditions, these are usually due to meniscus forces between a layer of adsorbed liquid (often water) on the surface of the sample and the tip. However, even in clean conditions, other attractive forces like Van der Waals will generally be present. These attractive forces move the resonance of the cantilever downward in frequency. Since the cantilever is being driven at the original resonance frequency of the cantilever, this will cause the cantilever amplitude to decrease as the attractive forces puli the resonance peak out from under the drive frequency. This decrease is often quite linear with distance, but doesn't have to be. In this case the tip is actually turning around some distance (this could typically be a few nanometers) before it touches the surface. This regime is commonly called the “attractive” regime because attractive forces dominate the behavior of the cantilever.