I'm not sure what this effect is called but i think it has something to do with track matte / tracking / masking. I'm super new to AE so forgive me for my amateur quesiton haha. Can I get help implementing this effect?
[Moved from generic Cloud/Setup forum to specific Program forum... Mod]
Try to provide us with more information. Maby show us exactly a piece of video with that effect or something.
Rotoscoping is kind of what this would be. (Tips for doing that more efficiently here.)
Basically, you need two layers.
Footage with the guy walking
Make a mask follow the guy out of the shot on the top layer, revealing the bottom layer. However, it may be easier to animate multiple masks, depending on the shot (thus the link I shared).