Swiping apps out of the recent apps list is vanilla, and yes, not well documented. This has been the topic of a decent amount of discussion on various Android forums… the consensus seems to be best described here in some comments: that the behavior is similar to but not exactly the same as closing an app — in general (for apps that don’t define explicit back button handling) it’s the same thing as hitting back enough times from within an application that you exit out of it.
The link has some more details on the specifics, but overall you can think of it as quitting the application.
Specific to the Music app, I believe it starts a service, so while the task itself (the Music app/UI) may be closed, the service continues to run in the background so that your music doesn’t suddenly stop just because the task got cleared out for memory management reasons. That may have affected what you saw.
I appear to have found the magical search terms that led to some explanations from Google employees. Specifically, I found a couple of different places where Dianne Hackborn explains what happens when you swipe something out of the recent list. The first is a comment on one of her Google+ posts:
[W]hat specifically happens when you swipe away a recent task is it:
(1) kills any background or empty processes of the application (see
for what this means), and (2) uses the new
API to tell any services of the application about the task being
removed so it can do whatever it thinks is appropriate.
She also notes in a blog comment:
Actually, removing an entry in recent tasks will kill any background
processes that exist for the process. It won’t directly causes
services to stop, however there is an API for them to find out the
task was removed to decide if they want this to mean they should stop.
This is so that removing say the recent task of an e-mail app won’t
cause it to stop checking for e-mail.
If you really want to completely stop an app, you can long press on
recent tasks to go to app info, and hit force stop there. For stop is
a complete kill of the app — all processes are killed, all services
stopped, all notifications removed, all alarms removed, etc. The app
is not allowed to launch again until explicitly requested.
So, it looks like the summary is that swiping an app out of the list will first kill all background processes for the app, then use
onTaskRemoved to notify the app that the background task was removed. At that point it looks like it’s up to the app to decide what happens, so I guess there technically isn’t a hard-and-fast rule about what happens to the app beyond that point.
There’s some info in the source code in the classes com.android.internal.policy.impl.RecentApplicationsBackground and com.android.internal.policy.impl.RecentApplicationsDialog.
If I read these correctly, there are specific handlers for selecting the apps but nothing special for swiping them except for
onDetachedFromWindow(), which calls
com.android.View.onDetachedFromWindow() which basically hides the element and clears it’s data. This would hint to the fact that nothing special happens on swiping the app, which corresponds with Austin Mills’ answer, because since the list doesn’t show the active app, the
onPause() and other system calls that are done when “quitting” an application have already happened.