I do that a lot at times, but I sometimes don't find it effective as cutting with a knife. I can relate though, but it's dissociate. I do it at times, mostly when I'm mad or frustrated, sometimes when I'm completely out of it. It's like an alternative I have if I don't have a sharp object at reach or grasp.

It's a another type of self injury for some people like myself to harshly scratch, bite, hit..ect. It could be out of frustration, anger, depression..ect. I thing if it did damage to your skin and legs or you spent a long time doing it in order to reach satisfaction of mutilation, it could be a relapse.