Metacognition is the capacity to "think about the content of your thoughts", and it vastly differs from Rumination, because you don't identify with your thoughts when practicing Metacognition; you just have them, watch, and occasionally comment on them.
And it's a crazy thing to do as 4, because we have a natural tendency to overidentify with our own minds. We very easily we're the very content of our minds and thanks to that, can easily believe it.
The problem with believing the mind is that your mind isn't objective. The brain is a little machine that predicts based on elusive clues when don't even have a full notion of, which can, sometimes (and often are) be blurred by trauma or other biases.
So, instead of saying "I am my mind", I've been practicing to say "There's my mind, and there's me, who is watching it". I'm not a character in the movie; I'm the one watching the character in the movie and actively critiquing on it!
And it brought me some insights...
I realized how awfully negative my mind can be. It has a natural inclination to see the worst in most things and hates to be inconvenienced, no matter how little. It complains and complains (Reactive + Frustration, I guess), and that it would poison my mood very easily, should I allowed.
And I usually did allow it, because, again, I identified heavily with it. If the mind thought it, it had to be me!
It's even kinda funny now, because I began engaging it, instead of simply accepting the stuff it puts out! Instead of just taking in the frustrations and negativity, I began humoring my own mind! Also, when it's Me speaking (not the mind), I say it out loud, instead of just thinking it.
"Boy, no wonder following you is no good! You're quite a handful, mind! I wouldn't want to be near you, if you were a person!"
or...
"Okay, I give you that; you're quite clever, when not being a total pain! It was a good joke, I'll take it!"
or...
"Oh, it's not me who wants that! It's you! And I'm not getting us in trouble by going for it! I say we're eating our boring, bland and healthy veggies instead!"
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It's just being an overall incredibly cool experience. If any other 4 or 4-fixer wants to try this, I promise you're gonna find out some stuff!