They have software that senses constant higher amplitudes/noise when no signal is applied and cut that frequency with a very tight Q to prevent influencing the stuff around it that doesn't need to be cut.
Since a tape deck, DVD, tuner, CD or Laser disc player can't be made to feed back, it's not something we need to look for in a HT or stereo system unless we'll be using it for Karaoke or DJ'ing. Without seeing data from the manufacturer, I would bet that it's a "brick wall limiter", which should be pretty self explanatory.
You're thinking of a limiter. The notch filters on a feedback destroyer aren't actively set, so if you get turntable feedback or bass rumble it won't save you.
Limiters will protect against that sort of feedback. I actually have limiters on all my channels except my rears (I will eventually add a unit there too). Where limiters are nice in home theater is you can set the limiters just below the clip point of your amplifiers, or where your drivers start loosing composure, and then you don't need to worry as much about a pop, cap discharge or someone too heavy with the volume from blowing your speakers.
I never hit my limiters in my home theater (especially not now), but it is a comforting thing to have. I have them all set to hard knee inf:1, so when they do hit them it is a brick wall.
