Just stumbled upon this question, and I find a very intuitive video about this.
Also, if you are interested, there are 2 legacy documents briefly explained its mechanic. (Just focus on the conceptual part, don’t get into the engine part.)
https://api.unrealengine.com/udk/Three/AdditiveAnimations.html
http://docs.cryengine.com/display/SDKDOC2/Additive+Animations