I have a few FN pistols, but they are the FNP and FNX models with a hammer. Never heard of this either.
Looking at the video they mention two conditions where it can fire:
Condition #1 - says if you press the gun against something pushing it out of battery and pull the trigger it doesn't fire. If you do that and hold the trigger back then pull back the pistol so it goes into battery, it will fire when it goes into battery. I don't know what other guns do when this happens. Never even thought of this scenario.
Condition #2 - I am not sure what they are talking about here. Looks like a they have a bullet in the chamber and are knocking the gun around in different ways and it goes off. It doesn't look like they are pulling the trigger. They hit it with a hammer (on the top of the slide) and slap it around in other ways. All they said was something about the trigger not resetting properly(?). Wonder if this was just a broken gun. Doesn't look normal.
I don't have an FNS to try any of this.
I wouldn't use a hammer though.