illegal Substitution or illegal Participation

[MD Longhorn]

Common sense says to blow it before the snap and penalize 5 yards.
Veteran officials say to blow it before the snap and penalize 5 yards.
And it just seems right to blow it before the snap and penalize 5 yards.

Problem is - in NCAA, the rules do not tell you to do that, and neither do the caseplays. The book tells you to prevent offense from running 12, but not defense. The other problem with calling it via common sense and not the book is that defense is ALLOWED to run their 12th man off the field if they can do it in time. So, exactly which second should we blow it dead?

Personally, I agree that it's not fair to give offense the 5-yard out instead of 15, and not the defense... but someone needs to put it in the book.