Normally to transition from one room into the next room you would need to open its door to walk into the transition. There are some doors in Zero Mission where it is advantageous to go through the door without opening it because it would require using ammo that you either don't have or are trying to conserve a specific amount of it. It is possible to use the Ice Beam to freeze and enemy such that Samus is stuck between the enemy and a door, and have the frozen enemy clip Samus through the door without opening it.
These clips are very precise and depend on the subpixels of the enemy approaching Samus, and they only work on doors that exit to the right side of the screen.
The most common door clip you will see is in the first Power Bomb room in the 100% run. This door clip has an RTA viable setup that guarantees a Space Pirate to be in the correct subpixel position every time, and it allows you to conserve one extra Power Bomb in this section and this enables a major route difference to avoid a shinespark chain later in the run.
The setup for this door clip is to jump up to the powerbomb pedestal just high enough that you ledge grab it, and climb up to collect the power bomb. Hold right the entire time so that you walk off onto the smaller pedestal, then off that one as well, and stop moving as soon as you land. While not necessary, it is recommended to tap A during the last fall off the small pedestal so that you can release right while spinning and automatically stop exactly when you land. If done correctly, this setup makes the Space Pirate walk into Samus and then forward a specific distance before trying to shoot. Once it starts the aiming animation is when you look up and freeze it. At this point crouching will force you very slightly into the door, and standing up again, by either pressing up or holding right, will clip you into the door.
Another door clip that has an RTA setup is used for skipping a red missile door for the 9% Hard mode category. This is useful because in Hard Mode you only get 2 missiles per missile pack, and this red door is immediately after a room that requires you to shoot two missile blocks with no way to refill your missiles afterwards (the exception is only on the PAL version of this game those missile blocks do not respawn and you can leave and come back after refilling), and this is the reason why 10% used to be the accepted low% category for Hard Mode for NTSC and JP versions of the game. The description in the video explains the details on how to make this setup work in real time.