I cannot comment on the legality of you being required to listen, only that the aircraft commander is legally required to make sure that the passengers receive a safety briefing. Sounds like BS to me though.
The additional brief you received would not normally form part of a pre-flight briefing (at least not at my employer), but it is exactly the kind of information you would receive in the event of an in-flight emergency occurring where there is time to prepare the cabin for the expected emergency. Part of this preparation would be the identification of Able Bodies Passengers (ABPs); passengers both willing and able to assist in the event of an evacuation being required.
In most circumstances, as an ABP you would be expected to hold other passengers back long enough for the crew to open the doors (and deploy the escape slides if the aircraft type has them) and then be the first passengers to evacuate, in order to assist the following passengers with their evacuation. The brief on door operation would be required in the event of crew incapacitation, along with how to operate the crew harness should you need to move the crew member away from the door area in order to evacuate.