There's more to "buy-in" of course than just hoping you have players who are predisposed to playing aggressively. It's nice if your players want to play hard and play fast, but that's not enough. I remember hearing a clinic lecture of Paul Westhead's some years ago in which he said, "Players claim they like to run, but few realize how difficult it really is to run hard all the time."
What Westhead was warning us is that players may buy in, but some of them won't stay "bought in" when the going gets tough. And what will happen when you have a tough game against a good opponent? Will their first instinct be to blame the System?
We had a saying at ONU (especially in our early System years): "The System won't work if you don't work!" It will be easy to point fingers when things go wrong, but the reality is that no style of play is a magic pill, and no system works when players don't play hard or execute well.
That's why initial buy in matters. If players don't have a strong initial commitment, if they don't want to play the System or don't believe in it, if they are unable to grasp not only how it works, but why it works, then they are going to bail on you when things get tough, and it's your task to convince them to hang in there. For lack of a better term, I call this educational process "Brainwashing." I want our kids to be zealots. They may have their doubts; your job is to answer those doubts and sell them on the merits of System basketball.
The same goes for coaches! Before you can sell them, you have to sell yourself. In my first System season (2004-05), I bought in all the way, but even then I had my doubts after a few early season setbacks. Here are the things that kept me bought in:
1) Grinnell's example. I knew it would work because I'd seen Grinnell make it work. There's nothing like having a role model, and I can promise you that if I had come up with the same exact ideas for System play that Coach Arseneault did, I'd never have stuck with them!!! Without a role model, without evidence that it would work, I know myself well enough to realize I'd have given up, and gone back to what I knew.
2) The numbers added up. I'm about as left-brain/analytical as they come, so I wanted some evidence from a statistical standpoint that this stuff would work. And Grinnell's "Formula for Success" made sense to me: 94 shots + 47 threes + 32 defensive turnovers + 40% offensive rebounds = Win! As a former math teacher, I looked at that and said to myself: "This isn't a random collection of goals. It's a concise description of how to go about generating more shots than one's opponent." Force the tempo, and get a lot of extra chances via offensive rebounds and steals. That's it.
This one insight revolutionized my understanding of the game, because after 25 years of hearing the mantra "You have to get better shots than your opponent or you'll lose!" now I saw there was another way to get the job done: Get a lot more shots than your opponent. Now I saw that my job as a basketball coach was to motivate and teach players how to get more shots via offensive rebounding and aggressive trapping defense.
That's when I bought in. I bought in because even though I knew I could also win by coaching conventional basketball (i.e. by "getting better shots")... I just didn't want to!
If you can convince yourself, you can convince your team. If you can't, then don't try the System. It won't work for you. YOU are the one who has to buy in.