Imagine an INFP, but so overly focused on the negative that they become fatalistic, critical of others (and even moreso of themselves than usual), and forget who they are (or at least who they perceive themselves to be).
We use our already weak Te and Fe to make judgements about others, our wounded Fi makes negative judgements about ourselves, and we back them up with our relatively weak Ti. Our Ni lets us know that the future is bleak, and all of those Ne "possibilities" are nothing, because we've already failed in so many ways that we can't come back now. Our Si reminds us of how we failed in the past, and our Se kicks into overdrive, making us do stupid things we'd never do normally.
It's like taking that giant bag of nastiness we always throw negative things about people and ourselves in, and letting it loose. Every negative thought, feeling, and action gets a chance to do its work. It's not pretty, but it has to happen in some form or another, or we'd explode. It's like your mind just can't handle anything else, and has to let this negativity out, one way or the other.
We INFPs can sometimes seem sweet and overly nice, but it comes at the cost of not letting our negativity out. Of course, when the shadow appears, it's not always this extreme. But sarcasm, little critical comments, and passive aggressiveness are part of the way in which the shadow of INFP shows in day to day conversation, unless the INFP is aware and tries to incorporate their shadow functions into decisions.