Board Thread:Breeding Questions/@comment-1158461-20130601215052/@comment-9946604-20130602012212

The only context I know that in is that it decreases your chances for rarer offsping. Ex. Wind plus chrystal has a better chance of a Pure offsping than Wind and Wind as they recieve the "negative effect".

Another example (don't know why you'd do this one):  Volcano and Flaming Rock are the two flame terra dragons. Volcano is more rare. You have a better chance of getting it with Volcano plus Flaming rock than with two of either. The easy way is flame and terra, but it's just an example.