As others have already suggested: I think it would be even better if the explanation box was fixed (e.g. top left corner of the screen) and had a FAT red arrow pointing to the area of the screen that it explains.

This would (imho) make it more obvious what's going on ("Ah, this is explaining that") and would improve usability.

But either way, this is a great start (I love the limelight-effect) and we are definitely going to build on this. Thank you for open sourcing it!