I've read in other threads that if you keep prospects in the minors for the first 20 days of the season you can buy an extra year of team control with them. My question is - is this tied to them being on the active roster or 40 man? In other words, would it be a moot point stashing my prospects for 20 days if they're already on the 40?

It's nothing to do with the 40. Team control has to do with salary arbitration. A player is in the majors on minimum salary till three years of major league service. Once a player hits three years major league service, he's arb eligible. So f he starts his rookie season in the majors from day one, he goes to arb at the start of season 4. If you delay him for the 20 days, he doesn't hit three years service till mid season 4, so he goes to arb at the start of season 5. Which also delays his eventual ability to go to free agency, thus "team control".

Gotcha, great explanation - thank you.

It is actually 20 ML games, not 20 days. With off days, 20 days is only 17 or 18 games at the start of the season.

I always go with the 20th game of the players minor league schedule, better safe than sorry.

FWIW, this is exactly what the Cubs did with Kris Bryant...kept him in minors until after the 20th game to save a year of service time. Bryant filed a grievance saying he was ML-ready and they only held him back to manipulate his service time, while the Cubs said he needed a little more development. The Cubs won the case.

I don't think the fake players will file a fake grievance.