Below the waist blocking is forbidden once you're outside of the tackle box.
That still doesn't stop them from their diving at your knees or ankles within the tackle box, or worse yet, having one blocker hold up the defender, while the other one dives low at his knees, making an impact a tenth of a second after the first blocker releases. That's still technically a cut block, and I can guarantee you'll see a lot of this from Navy this year.