If you are already in the park the day of the MVMCP that you hold a ticket for, you will need to go to one of the many stations that will be set up around the park to have your MVMCP ticket validated. They will then give you a wrist band to wear. At around 6 pm they begin directing people to get their wrist band for the party or begin making preparations to leave the park by 7 pm. Should someone without a ticket manage to stay in the park past 7 pm they will not be able to do anything. You are required to show your wrist band to buy food, buy souvenirs & get on any rides / shows. If a cast member sees someone without a band, they will ask them to leave the park immediately.
BTW, one of the things my son likes best about MVMCP is the Character Dance Party that is held @ Cosmic Ray's Starlight Café in Tomorrowland.