The idea is you go to the entrance of the ride when you would want to join the normal stand by line. But instead of getting in the physical line you get a return time. But unlike Fastpass or whatever, the whole rides capacity is dedicated to that. When they trialled this type of thing at Thorpe Park on your phone you'd have a countdown type thing which was the queue time. If the ride broke it would pause and tell you why etc. It never worked at Thorpe because they still had a standby queue too so you could double queue for that one ride.
It will work because it will be the only way from day one and people will have to use it. Express pass can still be possible, just their return time would be sooner. Again, when Thorpe did it they sold Fastrack alongside the reserve system. Think it worked out as 40% for standby, 40% for Reserve'n'ride and 20% for Fastrack of the hourly throughput.