If I am seeing right you suggest pulling down the bottom jaw or wiggling in the syringe. This is a bad idea. There jaw is not that strong and applying too much pressure can break the jaw. Wiggling it in can damage teeth gums and lips. You also don't want to grab around the body to secure as it can break ribs. The proper way is to secure the head by with fingers around the neck in a loop sort of so it is not applying pressure, but the animal can't turn his head. Then gently but firmly pinch the guler crest (the pouch area under is chin) and pull down and he will open his mouth nice and wide.
I don't mean to be critical, but some of the methods you show can cause harm with an unwilling chameleon. And as
@Beman says it is more critical to know why a chameleon is not eating in the first place. A healthy cham can go weeks without food so solving the problem is often a better course. Further most captive cham are over weight to begin with.
NOW THE GOOD NEWS!
I admire you for doing something. I have not had the courage to do YouTube video yet. It is not necessarily bad info just not the best as we have to consider the end user may have little to no experience, and these techniques are not the safest. I encourage you to continue but would suggest studying more on some of the top experts. like
https://chameleonacademy.com/ and
https://www.chameleons.info/en/ Or my site caskAbove which is sort of a cliff notes of the others . There is alot of misinformation out there, the more we can promote the advanced care and experience of these long term keepers and professionals the better. When you plan your videos set up rules and guide lines you will follow and write a script. This will help you produce consistent, informative videos. KEEP GOING we need people that care enough to make these videos.