You are correct about "out of phase". Reverse the +/- on one speaker only. It will not damage the speaker, it simply moves in the opposite direction of what was intended.
The idea with wiring one speaker out of phase and placing the speakers very close (I do as close as possible, without touching) is that the out of phase signal will cancel the in phase signal, therefore significantly reducing the perceived volume. You will be amazed at how much less volume you hear, even though the speaker driver is moving the same as if it were playing louder. Also, if you play a mono signal into both speakers at the same time, you will get a very low perceived volume because nearly ALL of the signal will be cancelled as you are playing an exact opposite signal through the out of phase setup. For a regular stereo setup, the "difference" signal will not be cancelled, ie, if an instrument typically plays from only one of the speakers during normal listening, it will not be cancelled because there is not an "opposite" signal being played from the other speaker.
BTW, the idea behind this setup is that you can have your speakers playing at relatively high volume, but because the signals are cancelled, it won't sound loud and you can play it continuously for long periods (like over night/during work) without waking up the household or the neighbors!!