You create a groove shadow in mixing by adding reverb to a beat and then placing it behind your main beat rather than directly applying it to the main beat which can create a more interesting ambience and effect to your beats.

You can watch a video tutorial below by Future Magazine and read more on how to create groove shadows here.

[youtube http://www.youtube.com/watch?v=1E67axz04C8&w=480&h=360]