Volley Secrets by Brett Hobden has been shown on Tennis Channel.
Brett Hobden discusses the three types of collisions (3 minutes) -
http://www.tennisresources.com/index.cfm?area=video_detail&vidid=2198&rv=1
Brett Hobden Volley Secrets (longer version, 19 minutes)
http://www.youtube.com/watch?v=fJb954_II7c
I learned particularly about the racket's 'loosing the collision' technique (racket head goes backwards) in these videos. I can easily do it in warm up but during a match my hard-to-change mind set rarely uses this very effective drop volley technique. In watching pro matches, you can use your Motorola DVR stop-action (
press the pause button again after it is stopped to advance one frame) to see that the pros use 'lose the collision' all the time usually for a very high percentage of drop volley winners. I don't believe that you can drop volley effectively unless you understand the 'lose the collision' concept.
It would be an interesting shot for TV broadcasts - to shown the drop volley in slow play and clearly show how the racket loses the collision and how very effective this shot is. Many - but not most - of my 4.0 opponents use drop volleys.