Hey, and thanks for asking!
Scrimba is a whole new format, and not a plugin you can install.
Technically, it records events instead of pixels (which is what normal video does). This means we have a DOM recorder that records all interactions the creator does when creating the screencast.
When you're viewing a Scrimba screencasts, the recorded events are replayed, so that you see exactly the same thing the creator saw when creating it.
This opens up for tons of possibilities, with interactivity being the most apparent one (jump into the editor and change the code). However, we've just begun scratching the surface of what we can do with this format, so stay tuned :)