The gist of the system is as followed:
When you pick up chastity, it will protect your X amount of damage. When you attack a monster, that chastity will become damaged. The next monster you attack must be weaker than the previous.
So, if you have a shield of 6 and enter a room of 11 (Jack), 6, 4, 8. The best order to attack would be 11, 8, 6, 4 for 7 damage.
If you had attacked the 8 first, your shield would break trying to attack the 11 since 8 > 11.