The switch undoubtedly lowers the frequency of the high pass filter which rolls off bass below the tuning frequency. Some EQ changes might also be made by throwing the switch. The high pass filter prevents the driver from trying to reproduce frequencies below the tuning frequency which might damage it because of excessive excursion.

Port plugging is what I have done with my JBL subs - they originally had 3 ports with a 25Hz tuning, and by plugging two of them, the tuning is around 12Hz. I lost efficiency which I made up for by using two subs per side instead of one.

I hate when that happens.