The requirement is that you be in fear for your life or the life of another in your vicinity. Let's say you grabbed a Bic pen, and went for someone's neck with it: in this situation the courts would favor the persons right to self defense given the credible threat to their life. The same counts for something that is a risk to incapacitate them, because courts have historically favored the argument that what happens after you are disarmed or incapacitated could result in death.

I'm not saying it's right, I'm just offering my observations of precedent.