Only when everything is clearly out of the tackle box would we want a foul.
I know you are only talking about a block by a qualified back, so forgive me if we expand the discussion to mitigate any misunderstanding such a broad sounding statement might cause, even though not at all incorrect.
There are several types of illegal blocks below the waist that can occur within the TB.
A stationary back within the TB at the snap is qualified to block below the waist FROM THE FRONT (only) in the TB, and while the ball is in the TB. For a qualified back, no, we are not going to be technical about the limits of the TB for a block FROM THE FRONT. A block, by a back, from the side below the waist is a foul, anywhere, anytime. A block below the waist, from any direction, by any back that is, or was ever, outside the TB, is a foul.
Linemen in the TB at the snap are qualified to block below the waist from the front or side, when the block is part of their ‘initial charge,’ which is a horrible label for what has been interpreted since we first got this rule change. We are all certainly OK with what has been interpreted, but a much better label would be the ‘initial engagement.’ Backpedaling a couple of steps and diving at an oncoming defender - even from the side - is not truly a ‘charge,’ (although it is considered such, by interpretation), but it is an ‘engagement,’ and that is OK. Truly charging forward, or backpedaling, before engaging is completely OK for a qualified lineman. Any block from the front by a lineman within the TB and while the ball is in the TB, is legal. Chasing a defender to then block him below the waist from the side within the TB, from any direction outside the TB, or from any direction after the ball has left the TB, is a foul. Again, if the question is the limits of the tackle box, we aren’t going to be too technical.
BBW is probably the most complicated rule in NCAA football. So many variables to analyze in real time. Penalty enforcement is complex, but we have time to ‘run the traps’ on that, and get help from crew mates. We don’t have that luxury with blocking below the waist.
And we didn’t even address BBW by Team B…