We have booked one night at RPR this July to enable us to take advantage of the fotl pass.
As far as I understand it, you can book into the hotel early on the morning of your stay. I would assume that your room will not be ready until 4pm that afternoon. However, as you have booked in, you will get given your room key/card and this is your fotl pass.
You can leave your overnight bag with the hotel (or in the car), and then go straight into the parks. You must have separate park admittance tickets - the room key/card is not an admittance ticket.
You can then use the fotl pass all of that day, every day for the length of your stay until park closing time on your final day.
I have read somewhere that you may be restricted to one fotl for each ride, but I think that may have been the Universal Express that you could buy in the parks (that has now been discontinued).
I think you also get preferential seating at the Universal restaurants, both in the park and City Walk, ie if a queue you would get seated first.
Obviously, if it gets too much during the afternoon, you can return to the hotel and chill out in the pool and then return during the evening.
I think this is right - if not, I am sure there is someone out there who will correct me.
