Accidentally downmixed stereo to mono, why does this work?
This can be easier to understand if you look at the waveforms.

In a push-pull, or bridge amplifier both lines are driven as shown below.



Notice \$L-\$ is literally the inverse of \$L+\$

Similarly the other signal, \$R-\$ is the inverse of \$R+\$

The difference between the +/- voltages is that excites the speakers.

Now, if you connect Opposites to one speaker the difference in signal becomes the mixture of both signals. Hey-Presto.. you have a mono system.

Note however, the amplitude of each "Side" is now effectively reduced by half. If you can't understand that, consider the case where there is no signal on the \$R\$ side. The difference between the blue lines is now only half what is was between \$L+\$ and \$L-\$.

If the original sound was recorded central, that is, an equal waveform on both left and right channels, the \$L+\$ waveform would be identical to the \$R+\$ waveform, same for the negatives. As such, joining  \$L+\$ with  \$R+\$ would result in no voltage difference for that sound. That is why you need to cross connect them.