You missunderstand, that was not planking, that was just ledgestallin, Plank didn't Plank *wow* once in the video. Idk Why, I'll try to example in details what planking is. Correct me if I'm wrong but......
The Metaknight is on the ledge, immediately while dropping from the ledge, he uses Uair, He still has invincibility frames from being on the ledge so he cannot be touched, Than immediately after the Uair and the move is over, He Up+B, Shuttle Loops back the stage. And repeat.
That is why Undr said you have to trust yourself being able to tech a shuttle loop, because if you try to stop it and you mess up your most likely going to get shuttle loop'd into the stage, and stage spiked. For some characters that's an 0-death. And for pit he makes him really vulnerable because he will most like have to go into Wings, and running from a Metaknight isn't easy, Esp. on a stage like YI.
So unless your like Lucario, and can run off, and Dair and Out-Prioritize the Shuttle Loop there's almost nothing you can do. Your probably thinking you can run up and grab the ledge on his Shuttle Loop and he dies, But the Uair when dropping prevents that and you'll get hit by Shuttle Loop when he grabs the edges.
Also his Name is Green because he's in BR, different from SBR, I guess BR *out-prioritizes* SBR.
Edit: Also I'm pretty sure marth can do it, and With a looped arrow, it'd be pretty hard to hit the exact time he's vulnerable. Plus it distracts and if your close to the ledge you could get fair'd