xkcd #3232: Countdown Standard
Title text:
Anyone who is caught counting ‘three … two … one … zero … GO!’ will be punished with a lifetime of eating only ISO standard food samples.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3232/
I think it’s so the person being lifted doesn’t tense, when ready for it. So you kind of surprised them but it’s an acceptable surprise, as they are expecting a move.
These are usually either unconscious or screaming people, not the kind to care about what an EMT says.