We had a little early information on the Project Hera task switching system before the announcement, but now things are becoming clearer as Googlers chime in with the specifics. At the I/O keynote, Google showed Chrome adding multiple tabs to the app switcher, but that's just the start of what's going to happen in Android L's multitasking.
First things, first – it looks awesome. The stack of cards is animated very smoothly and the way apps slide into place when you select them is great. Material Design apps also get title bar colors that match the app's dominant color. Beyond the visual appeal, the improvements to multitasking will allow all apps (not just Chrome) to have more than one card, provided they are for different tasks. Multitasking in L stresses documents, rather than activities (in the application sense). This leads us to the most underappreciated change to multitasking in L – the labels will always match.
If you're on Android 4.4 or below, try going to the Play Store and share an app to Gmail, Twitter, G+, anything really. The Play Store pulls up the app in question and you do your thing. However, if you hit the multitasking button, you're going to see an entry for the Play Store with a different app inside the preview (whatever you're sharing to). You can get pretty deep into another app like this, but the system still considers it part of the originating app's activity (in this example, the Play Store), so it stays under that single multitasking card. This is super confusing, but that's the way Android has always worked.
Android L changes that by tweaking the FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET flag. Even though the Play Store in the above example is launching the shared-to app, the L system goes out of its way to treat them as two different tasks. FLAG_ACTIVITY_NEW_DOCUMENT ensures that when you tap a link in an app that routes to a different app, that you aren't going to see an entry in the multitasking UI with the wrong name and icon attached to it. Additionally, when you go back to the original app, it should simply show the last screen you interacted with before the link. See the GIF above if you're still fuzzy. Even though the Play Store launched the Twitter share, it gets its own card in the stack. Going back to the Play Store is just the Play Store. It makes a ton of sense when you see it in action.