How to change clip with another clip?

Just seen Shaun of the Dead, and they are using it a lot.
Take a look at this scene:
www.dailymotion.com/video/xkz5mh_break-up_shortfilms
At 17.11, the door is closing, and when it does, it gives us another scene. Just like split screen.
They are also doing this when people walking by. A person goes in front of the camera, and when he passes, he drags the next clip with him.

What is this called? And can I find a tutorial out there?

It's a simple wipe, or an animated mask for more complex objects, that follows a person or object moving across a shot, revealing the next shot. It is one of the oldest tricks in the book. In the scene you have linked to they have added a glint over the wipe, but the technique stays the same.

Most important is that the person or object on which the transition is based has to cover the entire height and width of the frame while he/it moves. This is of course done during the shooting. Then you put a wipe or an animated mask over the object and you make it move at the same speed, revealing the next shot.

- Ronny

I think I understand the concept, but I don't know how to make it happen. Is there any tutorials you can link to?
Tried searching for transition wipes, animated wipes, but couldn't find anything.

I have searched for specific tutorials, but in vain. If the moving object has straight lines (like a door opening or closing) a regular wipe will do the job. If the moving object is a person you will have to create an animated mask using rotoscoping, which can be done in Motion. If I find some appropriate footage I will make a little video for you tomorrow.

- Ronny

I'm planning on doing a person wipe. Så the animated mask with rotoscoping is my favorite. Have heard of rotoscope, and I think I tried it once I got Motion, but I gave up. But now I have motivation, so it better work now

So if you make a video it will be very much appreciated!
Thank you for considering that, Ronny!

I will do it tomorrow morning.

- Ronny

Don't stress. Take your time, Ronny!