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Enter Clipping is a trick, that uses the "entering" animation most common with vehicles to skip triggers and pass through walls.
When you enter something, a vehicle for example, the player model is not actually passing through the area between yourself and the vehicle, but disappears, and is replaced by the model of the vehicle for the time you are using it. When it shows you entering the vehicle, it is only an animation, nothing more. For this reason it is possible to pass through triggers upon entering a vehicle, provided the width of the trigger is smaller than the maximum possible width between the player and the vehicle upon a successful "entering".
Using the exit animation, it is similarly possible to pass through walls, but this one is trickier. There are small areas surrounding the vehicle which are designated to be spots where the player is able to be exited, and are checked to make sure these places are clear before a player exits. If one of these places is blocked, it will force the player to exit into one of the other possible exit locations. If all the exits are blocked, it will not allow the player to exit until they are free. To trick the game into thinking an exit is free when it is not, it is possible to violently change the position of the airboat, usually via midair crash, upon exiting the vehicle near a wall.
There are other, simpler ways to exit through a wall, but these are dependent on the area. For example, in some places the vehicle naturally thinks an exit is free, but it is only by strange design of that specific terrain, and you end up being partway inside of a wall, after which you are automatically and instantaneously pushed to the other side.