All Android users know the window shade pattern, from the notifications shade that can be pulled down from the status bar. The window shade works exactly the same, but is applied in different situations.
The window shade is used mainly to provide navigational options. You can let the window shade contain shortcuts to popular parts in your app, or settings, or you can use it to let a user switch to a different section, which can often be seen in news apps. This option resembles the mode selector a lot, but the latter doesn't take over the whole screen. You can also use the window shade to let the user easily move to functionality, that needs to be accessed from everywhere in the app.
- Appealing way to let the user move to different content. The resemblance to a physical shade that has to be pulled down, or up, makes it fun to play with, and it looks nice too
- Doesn't take up a lot of screen real estate. Only the window shade control needs to be visible
- The window shade control can be overlooked when it's not given the right visual cue.
- The view that is covered by the window shade is not visible anymore, which could cause that the user feels lost.
- User could trigger it by accidentally trying to scroll a list/scrollview. (tacone)
- Some devices place haptic controls next to the bottom of the screen, so users may accidentally trigger a destructive action. (tad)
New York Times