I think I saw an interview on youtube with her, she has amazing voice control, its so difficult to get your breathing right if you do something like that. It amazes me she did it in 2 and a half takes, the instrument explanation makes perfect sense though. Probably why it sounds so natural as well.
Read an interview with her and she said she was singing words and the band said no, no lyrics. She said tried and it got to the point where she was going to give up as she just couldn't get anything, then it occurred to her to just be an instrument and that was that. After the first take, I think it was David Gilmour who said do it again with more emotion. By the third take it wasn't working for her, so she stopped and they got bits from all 3 takes to make up the final version.
She got paid £30 for the work at the time, but ended up successfully suing the band for royalties a few years ago, loss of earnings and stuff. She got songwriting credits or something, due to it being all her own work on the voice.
Christ yeah, hit squarely home
And then one day you find ten years have got behind you
No one told you when to run, you missed the starting gun
And you run and you run to catch up with the sun but it's sinking
Racing around to come up behind you again
The sun is the same in a relative way, but you're older
Shorter of breath and one day closer to death
Every year is getting shorter, never seem to find the time
Plans that either come to naught or half a page of scribbled lines