Proper use of Apply Additive?

I can’t get the Apply Additive node to work AT ALL, I was hoping someone here had an example of it in action or could at least explain what it does and how to use it.

Thanks.

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