In the image shown this is working by bracing itself against the top of the frame of the door, and the ground, creating a stable vertical platform that can not be removed without breaking the wall above the door, or if you were able somehow to destroy the floor.
The second aspect of how this design would work is that it is put on the side of the door that opens, meaning that someone trying to push the door from the opposite side would be pushing against the building itself (remember the vertical platform) And someone trying to force the door to open in the other direction would be stopped from doing so by the doors frame.
None of this specifically stops you from destroying the door, (IRL) but it makes destroying the door your only real option.
These exist in real life and are used by Military and Special Services (SWAT) to prevent doors from being used. They are a hydraulic wedge that fits under the door, it is then activated and swells from hydraulic pressure. forcing the door itself to wedge against the top of the door frame, turning the door itself into the vertical plain, and the wedge into the ground stop.
The pressure is generally so high that you can not dislodge the door with less force than it takes to break the door. And if it's a metal door (like those found in industrial/commercial zones) Then you're really not using that doorway for anything, and not getting through that way.
The object of the exerciser is not to make the door impossible to pass, but to make it impossible to pass quietly or quickly. A very effective way of 'herding' persons along a route that you have control of.