When an offensive player blocks a defensive player who is running towards his own end zone (from the side or behind).
This type of block below the waste has been illegal outside the tackle box since 2005. On March 20th 2013 the NFL voted to make peel-back blocks below the waste illegal inside the tackle box as well, resulting in a 15-yard penalty.
Defensive players generally can't see peel back blocks coming, making it nearly impossible to defend themselves. The play is particularily dangerous when offensive players attempt to cut down defensive players at the knees from their blindside.