Trying to work out how speakers which are silent from the rear work -- I.e. they have no bass spill out the back. I have recalled someone saying something about some clever technology, but I'm unable to find it.
The M3D sub you mentioned includes a technical description of the function directly on the product page.
A pair of 18-inch drivers is mounted at the front of the cabinet, and works in conjunction with a pair of 15-inch drivers facing the rear and is driven by a sophisticated phase manipulation circuit. The resulting directional pattern assures that very low-frequency energy does not spill onto the stage or cause excessive reverberation.
DoritoStyle's answer hit this squarely on the head, but I wanted to offer some more expansion on the concepts. Basically you put a set of speakers behind the main speakers. Since you know the distance from the front speaker to the back speaker, you can calculate the delay needed for sound to come out the second speaker such that it is the inverse of the sound just reaching it from the main speaker.
This inversion will effectively cancel out the sound heading directly backwards and destructively interfere with the sound heading out in other directions. By setting the speaker distance carefully, they can make it so that the amount of destruction heading backwards is not as bad as the destruction going forwards (when the sound from the second speaker reaches the main speaker with a constant frequency being produced.)
It is worth noting that outside of a relatively small frequency band, this technique starts behaving quite erratically as you move through frequencies that don't align as well from to the distance between the main and secondary speakers. You could probably process it such that it still cancels going backwards, but at certain frequencies, that will also produce varying levels of cancellation going forwards as well. You could counteract that variation by providing less cancellation at certain frequencies, but then you will get a really weird sound out of the back of the speaker, with some frequencies almost unaltered.
"Cardioid" low end speakers (usually subwoofers in my experience) are usually 2 (or more) drivers in a single enclosure which are intentionally fed signals that are delayed and out of phase with each other in a certain way to cancel out sound in the direction of the back of the cabinet.
This approach sacrifices power for directionality in order to reduce sound bleed to unwanted areas.
You can achieve similar results with individual enclosures with some processing.
Math to follow soon.