This isn't quite right.
The port loads the driver in its passband thus reducing excursion dramatically versus what the driver would otherwise have at that frequency and produces its own output via resonance. This allows you to have lower bass extension at a given SPL or higher peak SPL for a given bass extension for the same driver displacement (area * stroke), 3dB more efficiency out of the same box size or another 1/3 octave extension at the same efficiency, and lower IM and harmonic distortion for the lowest frequencies. More spouse friendly enclosure sizes, lower production costs, it looks like a big win.
However below its pass band the port acts like a leak and causes driver excursion to reach what it would be with no enclosure at all which in turn is much higher than with a sealed enclosure.
Where out-of-band signals are present this makes it _much_ easier to run the driver out to its mechanical limits where the voice coil former hitting the back plate can cause permanent damage.
Driver excursion quadruples (12dB/octave) to reproduce a given SPL with each octave lower a driver plays. As you drop below each of the two high-pass poles output halves (6dB/octave) so between the two excursion is only doubling with each octave lower for a given input level (with output 6dB/octave down) and below the lowest excursion is constant (with output 12dB/octave down).
With a sealed box and above the port's pass band you have the inductive impedance of the box's air spring in parallel with the driver's suspension compliance which increases the resonance and Q compared to having no enclosure; so below the port tune the same low frequency input produces more excursion than it would in a sealed enclosure. An electrical high-pass filter is prudent.
The other trade-offs that go with a port are a steeper roll-off less likely to be complimented by room (or cabin) gain so inside the sealed speaker might reach lower in-room and increased group delay (because loud speaker driver + enclosure combinations are minimum phase devices the steeper roll off causes more phase shift).
