Locked in started as everyday slang for being fully focused, then spread online as shorthand for switching into serious mode. Short voice versions like this one became popular in meme edits, gaming clips and sports highlights because the phrase needs no explanation. The existing caption locked tf in reflects the stronger slang form used on social media for emphasis. It is still used because the joke is flexible, working for real effort and for mocking someone who treats a tiny task like a championship final.