OK, from a sun-baked hillside deep in the mountains of Provence, here’s my best guess on the McCartney backing track scenario.
Any sensible director and performer would have a Plan B firmly in place for a show of this magnitude, and that’s what the backing track was for.
They need it in case a cold or other illness strikes on the day of the show or some other technical problem renders the “live” performance inaudible or unuseable in some other way.
Plan A is (surely, for a performer of Macca’s generation and experience) to perform it live.
Here’s how I would set it up.
The backing track would have a 2-bar click as count-in (maybe 1 bar, but 2 is safer) and this is fed to the drummer when the signal to start is given. The click is on its own track, and the stereo track with the backing music itself should be muted unless it needs to be used.
When the start cue is given, the engineer hits “Play,” the drummer hears two bars of click, the second of which he clicks his sticks with to count the “live” band in. They then play the tune in time with the click (although the track remains muted,) and in case of any problem arising the track can simply be unmuted.
What could possibly go wrong?
This: the engineer mutes the click. He realises it 1 bar in, unmutes it but in his panic also unmutes the backing track. The drummer counts the band in but is a bar behind because the first bar of the click was muted. the backing track starts in the “right” place; after two bars of click. For a few bars both the live sound and the backing track are sent to the front-of-house and TV/Radio feeds. The engineer takes a deep breath, assesses the situation and mutes the backing track. The band carry on playing live.
I wouldn’t have wanted to be the engineer at the post-mortem…