In my opinion, it all comes down to next Tuesday. But if things go well, and the road to blocking Trump's hostile takeover of the party at Cleveland is still open, here's the way it would work.
ADDENDUM: Michael Medved has commented that "brokered convention" is something of a misnomer, and he's right. There would be nothing "brokered" about it. The term that has been used in the past when such a possibility has arisen has most often been "open convention," and that's a much more accurate term.
I don't know that the paranoid muttering among Trump supporters that it would somehow be illegitimate or the result of an evil conspiracy of some kind if Trump were not the nominee of a convention a majority of whose delegates he failed to win is responsible for the sinister-sounding term "brokered," but what we'd actually be talking about would be a situation in which the unbound delegates to the convention simply made free use of their best judgement about who the nominee should be. The idea would be to find somebody everybody could rally around- which would certainly not be Donald Trump! In my opinion, it also ought not to be Mitt Romney, whom some have suggested, because so many Republicans see him as a symbol of something or other.
It could be John Kasich, whose entire strategy has been aimed all along at such an eventuality. More likely, it would be somebody new and fresh to whom nobody has any particular objections, almost certainly somebody who didn't even run in the primaries and caucuses.
Which could make it a very interesting week, if things (let us pray to the Lord) turn out that way.