Three separate studies analyzed the best way to recover from a hard workout or race! 
It's not resting, lying in bed, or icing down; it's working out passively. Here is an About-dot-Com article regarding the reasons and the research. From the article:
One study published in Medicine & Science in Sports and Exercise found that active recovery immediately after the event encourages recovery and reduces muscle lactate levels faster than complete rest. After hard intervals, one group rested completely while a second group exercised at 30 percent intensity between intervals.
The active group reduced blood lactate levels faster and could achieve a higher power output throughout the workout.
Three studies are are cited with links within the article [Link]
