mixmkr,
Yes they will be dancing to the song. Here is the issue: Let's say we have video of several different sections of the song. Then, in the video editor, we add the master music track. Now, how do we align the little 10 second bit we recorded during verse 2?
The editor thinks he can do it just by scooting the video clip back and forth until it aligns with the dancing. And , of course, we can zoom in and inspect the master audio wave and the live audio wave.
I was just looking for a more automated way to do it and a SMPTE clapboard would have been the way to go, but it is too expensive and not needed in this case (they say). Here's how it works:
1-You make a copy of the audio with SMPTE on the left and audio on the right (or vice versa)
2-The SMPTE feeds the clapboard
3- At the start of the scene, the camera records the clapboard and has an exact timecode for aligning to the master audio.
That's a simplified explanation and I am no expert, but that's how movies sync dialog overdubs and other events.