So, it varies by state because everything is a certain degree of a shit-show here. In my state, you show up at the primary election site and you ask for one of two ballots. That's enough to "register" you as a Dem or Rep. It's pretty meaningless.
I know of one coworker who voted for Trump in the 2015 primary to "take down the Republican party from the inside." Great strategy, Steve. I'm guessing that some similar idiocy is what's being talked about here.