It's actually a very slick system. The headphones will follow the source selected on the right hand display on the head unit (labeled "rear"). If this source is different than what is selected on the left display of the head unit (labeled "front"), then it turns the back speakers (including sub) off, so that the rear speakers aren't distracting the people in back with the headphones. It's not just limited to the RES system. The rear passengers can also listen to the radio, CD, or cassette independently of what the front listeners are playing through the front speakers. You can even listen to two separate audio CDs by putting one of them in the DVD player. If you turn the rear system power off, all speakers will follow the source selected for the front (but you must have rear power on to use the RES).
It sounds complicated and it's confusing for the first couple of days, but after that you'll appreciate the great level of functionality it has. It integrates VERY well into the factory audio system. This is the main reason that we went with the factory installed RES instead of saving money by going aftermarket.