see above for a full response.
The basics of it are, once May 15th hits and afterwards. you cannot horde acceptances. You need to make a decsion on which school you want to go to of the options you have before you (if you're lucky enough to get into multiple places. Whenever a new option opens up, you need to cut it down to 1 school. So if you have 2 acceptances and 3 wait-lists, on May 15th, you need to narrow it down to 1 acceptance and 3 wait-lists. If you come off a wait-list after May 15th, you need to decide quickly, so then it would be 1 acceptance and 2 wait-lists.