Impedance btw.
Least amount of resistance to the movement of the cone when a net force is applied to it. AKA Smaller port, or right on the frequency of the box where there is the least cone movement will increase impedance rise.
Do you wonder why frequencies below the tuning of the box cause the sub to go wacko? Not only is it allowed to move air easily (Unloading), but it also allowed to have the most power throughput from the low impedance(Lower compared to other, normal, conditions.).