An overhung coil will always keep a portion of its length within the gap. This ensures that the motor enters displacement compression relatively slowly as the driver nears Xmax. With an underhung coil, the total length of the coil is always within the magnetic gap. This design type is usually associated with high efficiency drivers and motors made this way enjoy increased linearity, reduced distortion and higher efficiency. However, once Xmax is exceeded, the underhung motor becomes rapidly non-linear, as you might well expect. This is rarely an issue though, because the overall motor/diaphragm system is designed so that Xmax is not exceeded until other parameters are nearing critical levels as well. But underhung designs are often more expensive, requiring larger and more powerful magnetic structures.