RNC COO (and attorney) Sean Cairncross does an excellent job of explaining how an "open" convention would work.
Here’s exactly how a brokered Republican convention would work - The Washington Post
One thing is true about California's place in the 2016 process. If a candidate is going to get to or over 1,237 -- and Trump is likely the only candidate who can do so before Cleveland -- that cannot mathematically occur until June 7. There are important contests before June 7, but none looms over the process more than California does.
To this point in 2016, much of the focus has been on the delegate allocation process. Trump is winning that race and has built a fairly healthy lead. However, there is also a parallel delegate selection process; a process to fill those slots allocated to the presidential candidate through votes in primaries and caucuses with actual people. And in many cases those people -- the delegates selected -- are not necessarily supporters of the candidate to whom they are bound for one or more ballots at the national convention. The early evidence suggests that Trump and John Kasich are playing catch up and that the Ted Cruz campaign did their due diligence identifying and activating supporters and/or potential delegate candidates before 2016. That is to say that Cruz is organized and Trump and Kasich are organizing. All other things being held equal, one would rather have frontloaded that legwork and tending to already-identified delegate candidates than to be doing it on the fly while also simultaneously competing for votes in the remaining primaries.
