Looking back on Mack's last few years at Texas, I have thought Mack himself was a good example of his saying about BBs contained in the Box.
For example, when Mack got everything rolling (staff, recruiting, coaching and winning) he was successful and won a NC and played for another. The BBs were all in the Box.
However, when he decided to change his Offensive Scheme to a Power Rushing attack... he had a squad he'd recruited and designed for the Spread Passing attack. Huge mistake to think he could snap his fingers and make such a big change quickly in a single season. Mack just had a case of Saban/Alabama scheme envy after losing to Nick's team. Many of the BBs escaped the Box.
Another bunch of BBs exited the Box when Colt McCoy was injured in the Bama game and Mack didn't have a backup QB prepared to step in and play. Mack had tried so hard to run up statistics for Colt's Heisman campaign that he kept McCoy on the field far too long in far too many games when he should have been getting snaps and experience for Garrett Gilbert. But he didn't... and Gilbert wasn't ready to step in behind center when McCoy was injured.
After those two things, the BBs were scattered and Mack's ego got in his way to the point that putting them all back in the Box looked like the outsized job it clearly would have been. By that time, Mack should have done the honorable thing and resigned.
I thanked him for the good times and forgave him for the bad times.