How does it work? You show up early, they let you in.
Seriously though, each day of the week one park will open an hour earlier than the posted hours. Extra Magic Hour applies exclusively to those staying at WDW resorts. CMs WILL often want to see hotel keys from those waiting in line, so it's not worth trying to fool the system if you're staying off-site.
WDW transportation will be running to accommodate early entry.
Here's the current schedule for EMH days:
Sunday - Magic Kingdom
Monday - Disney's Animal Kingdom
Tuesday - Disney-MGM Studios
Wednesday - Epcot®
Thursday - Magic Kingdom
Friday - Disney's Animal Kingdom
Saturday - Disney-MGM Studios
The park that has early entry on any given day does tend to be busier as the morning progresses. So if you don't plan to use early entry, you're better off choosing another park for the day. In other words, on Sunday, if you want to visit the Magic Kingdom, be sure to arrive early or you'll already have the crowds ahead of you.
You won't find ALL of the attractions open at each park for the early hour, but the major ones will be.