BBW (Sorry for this but...)

[dvasques]

I know I use NFL rules and this is not an NFL rules forum but I tried this question at the NFL rules Forum from RefStripes but got no answer.

I know it's forbidden to block bellow the waist on kicking situations. But I'm having a hard time clearing out when it is legal outside close line play. I've got one guy complaining that on a couple of his team's play, his defense on the field, the offense has a sweep. The outside LB goes for the runner but is blocked below the waist. Now, I can't find out when that would or wouldn't be a foul.