The contract states that if you don't make the room-night minimum, they will charge you the price of the cheapest room in your block for each of the missing nights.
However, I have *never* heard of anyone actually being penalized this way because Disney will work with you on the number of nights required if there are extenuating circumstances (small group, everyone lives in Orlando, etc.). Also, as the previous poster mentioned, you can add anyone's Disney resort reservation number to your block in order to qualify, so if there are other people you know traveling to Disney when you are, they may offer to help you meet your minimum.