Two drivers share an acoustic volume of air within a single enclosure. The best way to take advantage of this alignment is to mount one driver facing outwards with the other driver inverted and facing inwards. The drivers are then wired so that they are electrically out of phase while remaining mechanically still in phase with one another.
Odd ordered harmonics are cancelled out by using this approach according to Vance Dickason. According to M&K who specialize is push/pull subwoofers claim that this approach cancels out even ordered harmonics. So take your pick.
Either way, harmonic distortion is reduced in that any anomalies or variations in the two driver's spider, cone or suspension characteristics are canceled out by the other driver's inversely proportional anomalies and variations. The sound is said to be as accurate and pure as it can possibly be with each driver "correcting" the other driver.
Optional designs include having the two drivers share the same acoustic volume of air while maintaining the more traditional look of having both drivers fire forward into the listening environment. Though this does not have the same harmonic cancellation effect, all other characteristics between the two alignments is identical.