We were in WDW this past May when they were testing out this program. I would guess they received enough positive feedback that they are now making it permanent.
As has been said, each day there will be EITHER an EMH morning park OR an EMH evening park. The EMH mornings will work exactly the same as they do now - you simply have to show your resort ID when presenting your passes to enter the parks one hour early.
If the EMH evenings are managed the way they were in May, then they are similar to special evening events in that resort guests must go to specified locations within the park and show your resort ID to receive a wrist-band. As I recall, there were several locations available for this, so it was not inconvenient.
The week in May that we were at WDW, I believe there were 4 EMH mornings (one for each park) and 3 EMH evenings (one for all parks other than AK). Although they say "up to 3 hours", we know that MK and MGM were open a full 3 hours later on their respective EMH evenings when we were there.