Very simply put, if one places a block of some material on a level surface and pushes it horizontally, a certain coefficient of friction is measured. If one tightly couples an oscillator of some sort to the block such that it vibrates, a much smaller coefficient of friction is generally observed. Any explanations of this effect, with varying frequencies of oscillation? Worth noting is that the vibrating coefficient of friction is less than the normal dynamic coefficient of friction. While perhaps not more efficient energy-wise, it appears it could be a useful effect.
This effect is significant in debris flows, where large numbers of huge boulders move like a fluid. For a technical discussion see:
For a spectacular demonstration: https://www.youtube.com/watch?v=51C7vEAVbxk