What extending a song really means
Songs written in a rush often stop before they feel finished: a verse and a chorus, a loop that begins to groove, a theme that needs a bridge. An extender listens to what you have and writes the next section in the same key, tempo and sound, so you are not starting a second song from scratch.
The page works from the clip you upload and the point you mark. That point is the handover: everything before it is context, and everything after it is generated.