The invisible woged is an involuntary irrational emotional response. It is a physical representation of the fight or flight response. Just like every one does not react to danger. That is why all wesen do not woge when finding out Nick is a Grimm. For wesen to tap into that extra strengh. They have to do a full woge. Just like we have to pump our adrenaline.
Embrace your inner Biest..... We all have one