Just to give an example of how I've used it in the past:
We typically rope drop, then take an afternoon break, then go back in for late afternoon / early evening time.
So let's say you're using LL's in the morning, and you have lunch at 1pm. Your plan is to go back to the hotel after lunch. Let's also assume your last LL before lunch is at 12:30, and you tap in right at 12:30, and book Guardians at DCA for 6pm later that day. I'll set a timer for 2 hours, and then at 2:30 I'll book another ride - any ride at all. Just start the clock. I'll set the 2 hour timer again, and I might do some mindless refreshing every so often while I catch up on whatever else I want to on my phone just to find the ride I want.
At 4:30 I book my next ride, and let's say my plan is to go to DCA at 5pm.
When I enter DCA, I'll have three rides lined up: - Guardians at 6pm (in this example), whatever ride I ended up with using my 2:30 timeslot (even if I've changed the ride or time by refreshing and modifying), and then whatever ride I booked with my 4:30 timeslot. So in this hypothetical world it's something like:
5pm - arrive at DCA
5:30pm - ride booked using my 2:30 timeslot
6pm - Guardians
6:30pm - dinner
7pm - ride booked using my 4:30 timeslot
etc.
And of course I can keep booking/modifying for the rest of the night.
The other way I use stacking is to use it at rope drop. As soon as you tap in, even if you don't have early entry, you can book something. Book anything at all and don't plan on using it until later - you're just starting the clock, and you're gonna modify at some point.
So let's say I'm at
Disneyland and I tap in at 7:30am. No early entry, so it's an 8am rope drop and my plan is to do Fantasyland, then make my way to Matterhorn, then go to Space Mountain. At 7:30am, I book my first LL for the other side of the park - let's say it's Indiana Jones. I get an 8:30am timeslot, but I don't want to use it at 8:30 - so I keep an eye on the times during the wait for rope drop and push it back further in the morning.
It's 8am and I go about my business, do some Fantasyland rides in the first 20-30 minutes, and then I head to Matterhorn around 8:30/8:45. I check in on my Indy ride, and I can push it all the way to 11am. Fantastic. I do that, then go on Matterhorn, and then Space Mountain. Let's say I'm off Space Mountain by 9:30am - it's time to book my next ride - let's book Big Thunder for sometime in the late morning. Now I make my way to the other side of the park, try to hit up Jungle Cruise and/or Pirates while waits aren't terrible, maybe grab a snack, and now I know I've got Indy and Big Thunder just waiting for me to use.
So TL;DR - *if* you're the type to rope drop and tap in around 7:30am, then you can get an additional ride "stacked" even if you don't actually use the LL early in the morning.