I think it's the aspect that at that point it time it's physically impossible for those intrusive thoughts to come true.

At night when I'm alone, the intrusive thoughts (whether they be about stabbing, strangling, etc) aren't a bother simply because I'm alone. Sometimes I even welcome them. At that moment there's no one to hurt. Put me in a public place where many of those thoughts can be acted upon, and we have a problem.

Some days when I'm depressed or angry, I simply don't care about my life. On those days I might as well not have OCD, simply because I let the thoughts run rampant, not caring if I arbitrarily stab someone with my pencil.