Glad you like it. Actually it wasn't me who implemented it, it was my teammate Apy, but he described it as:
- take screenshot of first scene and second scene
- add them into a little transparent film reel png clipart
- then scroll
It scrolls up or down based on whether you're progressing to next level or rewinding to beginning of the same level. The scrolling is just done by translating the png up or down in code.