How they work great together is that you can go get a return time for a ride near the ride where you have a FP+, ride the ride you have the FP+ for, then go and ride the ride you got the DAS return time for. The return times with the DAS are 10 minutes less than the standby time. As an example, here's a situation my DD and I use often. We want to ride both Splash Mountain and Big Thunder Mountain. I pre-book a FP+ for Splash. We walk back to where the rides are, but before entering the FP+ line for Splash, I go over and get a return time for BTMRR. The standby wait for BTMRR is 30 minutes. Our return time is 20 minutes from then. We go ride Splash using our FP+, and by the time we are done, the 20 minutes has passed and we can go ride BTMRR with the return time. Or if the BTMRR standby wait was longer and our return time isn't ready yet by the time we are done with Splash, we can go grab a Mickey bar and enjoy that in a shaded spot while waiting for our return time. (And Mickey bars as a time filler might happen far more than I'm willing to admit. Those Mickey bars really hit the spot on a hot day.
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Other good tips for using the DAS: Get a return time for a ride before stopping to eat a meal. If you take a mid-day break, get a return time before leaving the park for a break. The return times are good until park closing that day so you aren't limited to an hour like FP+. So you can use your return time when you get back to the park after your break. This works well for my DD because taking a swim break at the resort helps her de-stress.
The DAS return times will only work for rides the person the DAS was issued to is riding. So if that person decides not to ride a particular ride, you can't use a DAS return time for that ride.