Howdy!
This is a fairly straight forward cart for a horizontal screen transition effect inspired by this particular effect from Pokemon Fire Red
https://youtu.be/LnAoD7hgDxw?t=4m23s
The effect itself isn't exactly spot on, the black bars move slightly faster than the 'pixel-pushing' effect created by moving the screen data around. I'm sure it's got something to do with the length of the bits of data I'm copying about but not 100% sure what the exact maths would be for a perfect implementation.
Hopefully this will come in useful for some of you :)
Oh, and here's a little challenge for you, how would you go about re-creating this effect but with vertically? How about diagonally?