munchkinomicon and sacrificing a level

i had a question re: obtaining the m'con via "sacrificing a level" if someone kicks open the door and is hit with curse: lose a level, or if the bad stuff from losing to a monster is "lose a level" that wouldn't count as sacrificing a level right? the player would have to willingly choose to be a level say 3 instead of 4 in order to obtain the book.

i only ask because until getting this booster i hadn't heard mention of being able to sacrifice a level. if i can sacrifice a level at will then wouldn't i be able to avoid some higher monsters? could i as a level 4 choose to revert to a level 3 to avoid fighting a monster who would not pursue level 3 or below? just like i can discard my warrior class to avoid a bonus a monster gets if i were a warrior.

thanks in advance.