They do wave releases, like the upcoming one, which mostly (entirely?) go through public playtesting at some point. They also do Super Secret Exclusives, which do not. Both types of release are hidden from most of us, but then the design thread is made public when the unit is released so you can read about the process.
There are also the public design threads, with two public designs going at once. Everyone is welcomed to participate in these. The public design process is otherwise the same as for other releases. You become eligible to have your design used as a public design by earning points (by playtesting) and winning the vote.
There's probably a post somewhere that explains that, but I thought I'd just go ahead. Is that about right?
I don't post often, so I'll take this opportunity to thank everyone involved.