Well thats at least what I think I got. We were practicing a no gi (although it can also work with the gi) guard pass against the seated guard. It involved collapsing the other persons upright shin with your own shin to keep their legs pinned and passing to the their back. It hurt a little in class because my sparring partners all had very bony shins (as do I) so it felt almost like a baseball bat. The next day is when it really started to hurt. My shin bled in class but I thought it was because of a graze which was the case the last time my shin bled.