Self injury releases endorphins, that is how it's similar to drugs... that is one theory anyway.
PMSelle, about the difference between self injury in spectrum and non-spectrum people, I can't pretend to be an expert; it would be fascinating for it to be studied though. My guess is that in the spectrum cutters/ self-injurers, there will be a history of self-injury going back to early childhood, whereas in the NT cutters there will be a PTSD trigger of some sort, like sexual abuse or some other abuse or trauma, at a later point in life, or an onset at puberty. One thing that always mystified me about my self-injury is that I've done it all my life, as far back as I can remember, and I never understood why until getting a plausible reason by learning about AS. As far as widely defining "stimming--" I don't think it's extraordinary to say that stimming can be self-destructive. Some people hit themselves with hard objects, or pick their scalp or skin until they bleed, and of course there is head banging. You might or might not be using cutting as a violent form of stimming... but if you cut to self-calm, that does fit one of the purposes of stimming. But instead of banging, biting, or hitting, you're using a razor. Just a theory, I absolutely can't pretend to know your reasons, but for me it made a lot of sense.
Another thing that might be a factor in spectrum self-injury is sensory overload in addition to emotional overload, vs. solely emotional overload.