It was explained to me that the problem is pumps - the very fastest roller coasters use powerful electro magnetics to power them which would kill all switched on electrical equipment in the vicinity, including insulin pumps. So the pump has to be removed and left with someone who is not going on the ride, but the pump user can't then stand in a queue for 2+ hours without their pump. So you get a queue jump pass and then only wait max a few minutes each time!
I think for the most popular rides they have a half hour time limit between them, so that you can't have 10 goes before all those poor people queuing have even had one. Daughter isn't big enough for the really fun stuff yet (can't wait until she is

) but once you've got the red armband the ride operators aren't going to ask why you've got it, they just let you on. So all the stuff we went on was probably pump safe and had smaller queues but it still saved us some time. We had about 4 goes on the runaway train back to back
To get the pass we have a letter from our DSN that we show at guest services. That states that standing in queues for long periods can cause blood sugars to drop dangerously low, which is clearly rubbish but I guess the average Joe wouldn't understand all the pump stuff - and when we went we were with a group of other D kids so they hardly bothered reading it at all, having already seen several!
We used it at EuroDisney as well. Can't wait until daughter is big enough to go on the really fast roller coasters as well, then we can jump the really big queues!