Either the lowest-ranking candidate's delegates get unbound, or the low-rank candidate to whom the delegates are bound instructs them who to vote for if he can't win.
Listening to Hannity on the radio interviewing Cruz.
Hannity..You have called on John Kasich to withdraw from the race as he has no pathway to 1,237. If you reach a point where you can’t reach 1,237 before the convention will you withdraw from the race?
Cruz...I have a pathway to 1,237 on the second ballot. Donald Trump can not win the general election without republican voters and I will win the nomination at the convention on the second ballot. Donald Trump can not win the nomination.
Hannity..Are you saying that if Donald Trump arrives at the convention with 1,236 delegates that he should not win the nomination?
Cruz...If Donald Trump arrives at the convention with 1,236 delegates then he will not be the nominee, I will win the nomination on the second ballot. If Donald Trump can not win the delegates with republican voters before the convention he can not win the general election and I will be the nominee.
Odd that with all that Ted Cruz doesn’t explain how he can win the nomination and general election by coming to the convention with far less support from the voters than Donald Trump if Trump arrives with 1,236 delegates which would mean Cruz would likely have far less than Trump.