He did not have to reapply. Caltech's stance is once you're admitted, you are welcome to come back and keep trying. I know one who came back multiple times. Your credits don't expire, either, like they do other places. It's one of the great things about Caltech. The administration was just made up of nice people.
I can only speak for the process that was in place when I applied back in the 70s. I sent them a letter asking for an application, which they sent to me. I filled it out, and sent it in with a check for the application fee. Nobody helped me with it, nor was I coached.
They had some specific requirements, like you had to graduate from high school, but I knew one person they waived that for.
What they looked for was a triad:
1. high school success
2. high SAT scores
3. evidence you're a highly motivated person
You had to have all three. And it worked for Caltech - being around smart people can be fun, being around motivated people can be fun, being around smart and motivated people means you're going to have more fun than you ever thought possible.
Caltech wasn't a place where you talked about football. If you wanted to have Carl Sagan drop by the dorm and talk about silicon giraffes, that was what was fun. If your room was next to Hal Finney's, that was fun. If a couple doors down was a guy who designed and built a working computer out of NAND gates, that was fun. If the guy 4 doors down built a powerful amplifier, and set up a giant set of speakers at the nodes of the hall and hooked it to a signal generator to emit sound below the hearing range, but was at the resonant frequency of the building, so it soon would start to go whump whump whump ... hahaha I was never bored for 4 years. Never.
I'd be a bit careful about optimizing for it. You really have to want to go there, and for the people who thrive there, it just is who they are. Caltech just wanted to be sure that the ones they admitted would be a natural fit, not an optimized fit. If you're not a natural fit, you won't be happy there, and will drop out. Nobody wants that.
I was told years later that they were skeptical about my application, and so sent someone out to interview me. He couldn't get a word in edgewise as I went off on all the projects I was working on, showed him diagrams of them, etc. He told the committee he thought I was worth taking a chance on.
I was rejected by MIT, Harvard, and Stanford, despite being legacy for MIT and Harvard.
> Thanks for making D, by the way!
You're welcome! Working on D is a great pleasure of mine. The way Caltech was run is how I've tried to run the D Language Foundation. In many ways the DLF community is our very own Caltech :-)