If you interviewed at 5 schools and have an admission at 2 before match day, you can only hold one admission after match day.
You can rank the 3 schools you have not been admitted to above the one admission you can keep. They keep swapping the schools until June 15th (not sure if it is same date this year since some dates have been moved) when the school you hold becomes your final school.
So if you have schools 1-5 you interviewed at and admitted to schools 4 and 5.
You rank your schools same order on match day even though you are not admitted to 1,2,3.
If no one is admitting you on match day, now you are waitlisted at 1,2,3 while holding an admission at 4 but lose admission at 5.
February 15th, an opening opens up at school 3 - you are switched from school 4 to 3.
April 30th - an opening opens up at 2, now you are moved from 3 to 2.
June 15th nothing has changed - you plan to attend school 2.