I was wondering what happens if you generate a key, and then send coins to yourself on that key, all within the same wallet.
A transaction is created, broadcast, and eventually included in the blockchain that transfers control over that value to that new key.
Let's say you are using Bitcoin Core, you generate a key, then you use coin control to send some coins you want to de-attach from an address into a newly generated address, is that a good privacy practice?
It won't increase privacy very much. It will be easy to see in the blockchain which outputs were spent and which address was used.
It's like a cheaper mixer with only "1 pass" protection.
Since there isn't any mixing, it isn't a very good mixer.
I don't want to use mixers, because im scared to get "tainted coins".
How do you know that they coins you have aren't "tainted". They very well may have been "tainted" before you received them.
All of my coins are legal and im not doing anything that worries me, other than personal privacy. I don't want to mix my coins with other coins that i have no idea where they come from.
Since you have no idea where the bitcoins came from before they were sent to you, your coins are already mixed "with other coins that i have no idea where they come from".
And I just wonder how does this work. The transaction happens just like a regular transaction if you send it to yourself I guess?
Correct. The network and the blockchain don't know anything about what wallet has those keys. It doesn't know and doesn't care if you are sending to an address in a different wallet or the same wallet.
or is it faster somehow?
No.