Override this to intercept key down events before they are processed by the application. If you return true, the application will not process the event itself. If you return false, the normal application processing will occur as if the IME had not seen the event at all.

The default implementation intercepts Android.Views.KeyEvent.KEYCODE_BACK if the IME is currently shown, to possibly hide it when the key goes up (if not canceled or long pressed). In addition, in fullscreen mode only, it will consume DPAD movement events to move the cursor in the extracted text view, not allowing them to perform navigation in the underlying application.

[Android Documentation]