A "brokered" convention is what happens if no candidate wins a majority on the first ballot. After the first ballot, delegates pledged to particular candidates are released from their pledge. On subsequent ballots, they are free to switch their votes until some candidate finally wins a majority.
So, what happens if the top two are Trump and Cruz or Cruz and Trump? I would think the lower vote-getter would ask his delegates to vote for the higher, who would then win on the second ballot.
Of course, many more chaotic outcomes are possible.
Actually, in that case you would have Trump and Cruz asking the delegates for Rubio, Carson, etc for their votes. I don't think either of them would drop out in favor of the other if the vote was fairly close on the first ballot.