essentially, in a sealed box or free air, near resonance, it is difficult for the amplifier to provide more power to the woofer. the woofer is efficiently moving on its own, and the mechanical energy is being returned at the same time the electrical energy is being provided. all in all, the electrical system is providing very little power to keep the speaker moving.
from math, we know that if we know any two of voltage, current, power, or impedance, we know the other two as well. in this case, we know that the amplifier's output voltage is not changed (that's how the amplifier works), and we know that the speaker is drawing much less power. thus there is less current and a higher impedance.
while this efficient states sounds nifty -- lots of SPL with little power -- it has one large drawback. In order to make sound, the speaker has to move. With a ported box, the speaker doesn't really need to move IF it can move air with the port. the ported box ends up with a low excursion, low impedance, high efficiency point, and that allows the user to dump a lot of power with less concerns of overexcursion to get large gains.
