It's pretty simple, you interview at a program, the PD really likes you, hands you a contract and you sign. You don't go through the match. Pretty much anyone but non-4th year MD students can do this. Starting next year the NRMP is changing the rules and there will likely be a huge decrease in the number of prematches offered. The NRMP is requiring that a program either offer ALL their slots in the match, or don't go through the match at all. If you don't go through the match, you'll only be getting IMG and DO students which isn't an ideal situation for most programs.