I'd imagine plenty of people have figured this out, but the ext source button works like a number pad when recording a sequence. the actions of both turning the pad on and off are recorded into the pattern. Thought this might be handy for some, as you can play a clip through the RCA-in during pattern playback, and a different snippet of sound will shine through with every loop of a pattern that contains a hit of the ext source button.

Sorry if this is old news, I just never saw this in the manual when I got my SP!