I don't understand. It's pre-recorded, but they sing live?
It's a huge stadium so in order for them not to be drowned out sometimes the people behind it might ask them to record the track to play while they perform so they can be heard around the stadium. In Mariahs case she totally let the track do ALL the work and she lipped the high notes and most of the performance. Whitney and Beyonce both sang loud and clear and the tracks if they were even playing weren't loud.