When I cut, sometimes I can cut deep enough to really make it bleed and sometimes I can only cut shallow. I think it has a lot to do with my state of mind at the time. The more emotionally worked up I get the greater need for a release, then I cut deeper. For me its about the pain and the blood. I need to feel it and see the blood flow, letting those pent up emotions out. The shallow cuts still help because I feel some pain and they bleed a little but its always best if the cuts are deeper and the blood flows.