Fig.97  Phase displacement triggers the shift of potential holes relative to their initial position, and appropriately, the shift of sources. The sources are influenced by the wave field so their natural reaction would be motion toward potential holes. If such motion is not obstructed the sources will be moving until they find themselves in the areas of stable balance.

For the sources to be again in potential holes with the standing wave anti-node between them, the system must pick a certain velocity V.