The turnstile master computer at each park keeps track of both entries and exits. When a park capacity closes (which is done in stages) there has to be a certain number of people exiting, with the total "in park" below a certain number, before they start allowing people in again.
I am low enough on the totem pole that I have no idea what these numbers are.
There are four stages to park closing:
Stage 1 - The parking lot entrance is closed. You can only access the park if you are using Disney Transportation.
Stage 2 - The ticket windows are all closed (and the automated Ticket Vending Machines are turned off). Unless you already have a ticket you cannot get in.
Stage 3 - Only people with Annual Passes or current Resort IDs will be allowed in.
Stage 4 - Nobody is allowed in.
Note - having restaurant reservations in a park, even with written proof, will not override a stage closing for you.
Also, if you are staying at a Disney owned and operated Resort and have a ticket, you are guaranteed entry into A park. Not necessarily the one you want (unless you want to go into AK). And if you have entered and left a park and do not have a hopper you may not get into anything.
