How do you pull the top of the handgun off like those moves in the movies or securty training?

That doesn't happen that often in reality.

I have a friend that is a police officer. When I was looking for a weapon to carry he advised me on what to get and what not to get. There was one semiautomatic that I liked. He told me not to get it. I liked it. It was bi-color and the price I thought was reasonable. I didn't want to spend a lot. He began to tell me why I shouldn't get it. He said have you ever seen one of those Jet Li movies where he grabs the gun slide and takes it off before that guy can shoot it? You can actually do that with that gun. It was a poorly design weapon. You don't want to have your life or family life dependent on that gun. Chance of that happening is slim and none, but it could. I think he mentioned that if you dropped it the slide may come apart. Of course I didn't buy it.

I wish I remember what gun it was.

There is no disarming training that teaches you to do that that I'm aware.

That can only be done with some poorly made and designed automatics which have a button or a lock that releases the slide, barrel, and upper part of the frame from the lower half of the weapon which is the butt and chamber. Automatics are basically constructed this way so that they can be cleaned more easily and thoroughly as well as sometimes helping to clear jams. Not all automatics have this button or lock in the same location so you have to be pretty familiar with the gun and model and doing this with one that you are not familiar with and maybe in semi darkness would not be easy to do so the whole "don't try this at home" thing should apply here.

Also how the person is holding the weapon becomes a factor in all this and if they are an amateur just holding the weapon extended out there. It is really something that is movie type hype and most guns you can't do this with that fast before they get a shot off with you standing in front of the barrel. I would never want to bet my life on such a foolish and impractical technique as this.

The topslide on a beretta chambers the round, if you hold this part of the gun in place so that it cannot move then the gun cannot fire. Same with holding the cylinder on a regular 6-shooter. Stripping the gun in the way your talking is best left to the movies. Normally this is only done when cleaning the weapon.

As others have stated, this is limited to just a few guns with this flaw, and it requires a level of precision you're not going to replicate with an uncooperative attacker. Learn real disarms, instead.

Most of this is theatrical and for movies and tv but on certan handguns there is a lever that you can use to release the top. Krav Maga is a good martial art to look to for weapon dissarming.