Basically, a push-pull subwoofer contains two low-frequency units, and the externally mounted unit projects sound to the outside space, and the unit mounted inside projects the sound inside the box. This design takes advantage of the air pressure that becomes the control of the cone from movement to rest. Moreover, the sound basin is "one positive and one reverse" regardless of the movement to the outside of the box, and the symmetry of the positive and negative half waves is much better than that of using only one unit, so the even harmonics are also lower. Such a positive and negative sound projection can eliminate the reaction force inside the low frequency unit, and can also greatly reduce the even (especially secondary) harmonic distortion. The push-pull configuration provides a higher purity output (other aspects are basically the same) compared to conventional projection placement subwoofers; however, when near the maximum output, odd harmonic peaks are produced. Of course, even harmonic components are still kept at a fairly low level. gree , https://www.greegroups.com
The cabinet is a closed structure without any openings. When the positive half-cycle signal is output, the front side of the upper unit and the reverse side of the lower unit are simultaneously pushed outward. At this time, the internal volume of the speaker becomes large, and the air pressure outside the box is higher than the inside of the box, and the air pushes the sound bowl inward. When it wants to output a negative half-cycle waveform, the sound basin movement is squeezed into the box. At this time, the air pressure inside the box is higher than the outside of the box, and the air pressure pushes the sound bowl outward. 
On the contrary, there is also an externally mounted low frequency unit that projects sound into the interior of the cabinet, and a unit mounted inside projects a sound to the outside room. 