Confusion in Case Book

[GPC2]

When looking for verification on the DB unsportsmanlike penalties, I came across a play that I cannot understand in the Case Book.

5.1.2 SITUATION B: During A1's run for a first down to B's 20-yard line, B12 commits a nonplayer foul for a substitute entering the field during the down, but not participating. Immediately followin the down, A1 taunts an opponent. RULING: If accepted, the penalty for B entering during the down is enforced from the previous spot. After the decision in this foul, A will be penalized 15 yards for unsportsmanlike conduct and A will have the ball for a new series, first and 10.

Now, for the life of me I can't understand how B's Illegal Substitution penalty will be enforced from the previous spot.