An audio clip from the mixing board from a concert of the younger Iglesias circulated a few years ago. He sounded horrible like a yelping dog. Apparently those acts that rely on elaborate dancing routines just yell the lyrics while out of the breath and in motion.
I don't know how they do it, but some how a cleaned up version is transmitted into the arena.
Howard Stern challenged Iglesias to come on air and prove he could sing. He came on the show and sang accompanied with an acoustic guitar. Great sport and sounded good on a small scale, though don't know how any of these 'performers' pull it off on a real stage.