Subwoofer with magnet sticking out of enclosure....questions?

theres usually another sub underneath it facing the correct way , theres a way to wire it if you going to run it that way also opposite of the other where they push and pull , i cant remember what they call it but it was popular in the early 90s i knew a couple of guys that did it and it sounded OK but its expensive and i believe its not very efficient. but its screwed in from the back just like you would if it was facing the other way. its a way where you can put more subsin one box