it's that voice in the back that's always there, but waits for a quiet moment to suggest crazy shit. I liken it to very very mild, controlled tourette's - random impulses that shouldn't even raise to the level of conscious thought but for whatever reason slip through the 'noise' filter.
It definitely is crazy shit, not always self harm. For whatever reason I always get it when I see nice cars, my brain immediately goes to "kick the headlights!" and then I cringe when I imagine it actually happening, it's so odd because the desire pops up but my reaction to it is disgust or "pain" like I definitely don't want to do it.