it's running a pair of speakers and sub from a 2 ch amp. When set up right it's not harmful to the amp.
Any 2 channel bridgable amp can do it.
IMO it's rarely useful as the sub doesn't get much power, plus you have to use a passive crossover on the sub. It's just a hassle for not much performance.
Listing "tri-mode" in the sales specs of an amp is like listing "rust-proofing" on a car.