The Russians call it "imitatsiya" - shadow swinging in order to properly reinforce a neuro-musuclar pathway. They use it a lot!
I love shadow swings and frequently ask my players to shadow (especially with their eyes closed, so they have to concentrate on the feel of the swing). I tend to use it most when correcting issues with swings, the moment the skill breaks down, they shadow the correct swing to reinforce the circuit.
I tweet - @ashtennis guru (no spaces)
I Shoot - www.flickr.com/photos/ashtennis guru/ (again no spaces! grrr)