This is a good section from the Slate article you quoted:

Now, tell me more about how bound delegates become free agents.

Exactly which ones will be freed and when depends on a rather complicated set of state party rules—which, like the national ones, are not written in stone—but by the New York Times’ count, the number of unbound delegates would grow from 5 percent during the first round of voting to 57 percent in the second, and then to 81 percent in the third. Even those are rough numbers, though, since states can unbind their delegates if the candidate they were assigned to vote for withdraws from the race or fails to meet certain vote thresholds on the convention floor.

Wait, but why would a Trump delegate defect to another camp? Trump fans don’t seem like the type of people who change their minds about Trump.

True. But while the bulk of delegates arrive in Cleveland bound to a particular candidate, that doesn’t mean they necessarily personally favor that candidate. A Trump delegate could very well loathe Trump; a Ted Cruz delegate might prefer John Kasich. And so on. Some states allow candidates to select the delegates that will represent them—making them more likely to remain loyal even once unbound—but those delegates only account for about 14 percent of the total. The vast majority of the rest are selected during state or district conventions that are held well after the actual state primaries or caucuses. Those slots tend to be filled with rank-and-file Republicans who are involved with the state and local party, making them theoretically more open to the GOP establishment’s anybody-but-Trump entreaties than primary voters have been.

So once a delegate becomes unbound, who can he or she vote for?

Anyone else whose name has been formally placed into nomination—be it someone who is already an official candidate this year, like Cruz, Rubio, or Kasich, or someone who isn’t, be it Mitt Romney, Paul Ryan, or whoever else the GOP poobahs try to cast as a white knight.