Trainers go hard

Watch the actual video before talking crap about something you know nothing about. It talks about somebodies immediate reaction to a fist coming at their face, is to bend their head down to protect it, which is completely true. If you have any arm muscle to speak of, this will mutilate your hand.
The title is very vague, the general idea of the video is to use caution when punching. You sound like you've never been in a fight, so if you ever did, you would be one of those retards in the videos that lose their head and start flailing their fists as much as they can, which is actually extremely easy to do. When you get hit, a lot of people get overcome with rage (this is more common if you know the opposition), unless they're particularly meek or experienced. This makes them lose, because they take no time to decide on an effective method of attack or defence.
The video is explaining that, in a fight, you should fight your urge to immediately thrust your fist at their head, and use more refined techniques. If you know how to punch, by all means do it, but you have to spend time finding an opportunity to hit a vulnerable point.
In other words, you need to realize that you don't fucking know everything, especially when you haven't even watched the video you're talking about.

You didn't ask for an explanation, by saying it was stupid shit, you showed your only intention was to make fun of it.
The time you've spent doing a martial art doesn't mean anything if the student doesn't seem to be able to learn anything, as it seems to be in your case.

If you've been kickboxing for 4 years then you've learned nothing. You should know that a basic hand defense for a cross would be to tighten up, press your fist to your temple, and wait. This will absolutely destroy your hand if your punch doesn't land where you want it to. While I've never hear this tip before, it makes a lot of sense to me.

It hurts to be punched in the face, but their reaction is to lower their head, which means you're either hitting their forehead (one of the hardest parts of the head) or their chin (also another very hard part in the head) and you easily break your knuckles. Anybody that has experience in real fighting knows this.

Read my post above.