My $0.02....
Here's a video where TP&W talks about it and demonstrates the procedure:
https://youtu.be/pEeQrsHcwf8I was surprised that I had to fizz a 7-pounder a few months ago that came out of 15ft of water. I've caught them deeper and didn't have to fizz them, so I didn't check on her until weigh-in. So lesson learned there is to just regularly check on your fish, even if they didn't come from real deep water.
To clear the needle after each use and keep it from clogging, I keep the syringe in a bag with the needle and right after fizzing one, fill the syringe with water, attach the needle, and push all the water through the needle to clear it.