Board Thread:Speculation House/@comment-27883360-20160305074527/@comment-26397825-20161103051830

Or 4, it's a situational semblance and doesn't activate unless a specific scenario happens, like Nora's or Yang's. Nora's is only useful if she is subjected to electrical attacks, which is not something that really happens that often when fighting Grimm. Yang's is a bit more useful in that it needs her to be damaged inorder to activate, but it still needs that specific catalyst.

It's not too far fetched to say Ren could have a semblance similar to theirs in that it needs a very specific catalyst to actually activate.