I have a cheap Chinese Bluetooth audio amplifier like this:
It has four screw terminals for the speaker connections, namely L+, L-, R+ and R-. I accidentally connected a single speaker to L+ and R-, and was surprised to find that I could hear both the left and right channel over the single speaker. I decided to experiment and found out that if you connect the speaker the proper way (so either to L- and L+, or to R- and R+) it would only play the single channel you'd expect it to play, but if you connected either L+ and R- or R+ and L-, it would "downmix" both stereo channels to a single channel and play them both over the single speaker.
I've been trying to wrap my head around how that works, but I can't figure it out. Is the board more sophisticated than I expected and does it output mono when it discovers no load on the different terminals, or is there something else going on?