[Schultj]

The all-but-one principle relates to enforcement of all penalties at the end of the run "but one"; the offense fouling at a spot behind the resulting spot on the play.
Example 1: A1 throws an imcomplete pass, but during the play B1 holds. The result of the play is the original line of scrimmage, so the penalty will be enforced from there (unless declined).
Example 2: A1 throws a completed pass to the A25, but there is holding on B1 at the A20. This time the penalty would be "tacked on" to the end of the run, the A25 (unless declined).
Example 3: A1 throws a completed pass to the A25, but A88 holds at the A20. This time, the foul is on the offense, so the penalty is "tacked on" at the A20, not the A25. The offense gained an unfair advantage with the hold at the A20, so they won't be awarded with any more yardage from that point (unless the penalty is declined).