A Question on the Democratic Convention

For those of you who still sometimes come by, I have a question.

What is the rule for how delegates to the Democratic Convention in Denver this year are held to the candidate they are pledged to? In other words, are delegates held to their candidate until the candidate themself releases them, or is there some releasing mechanism like, say after the first or second round of voting, they can vote their conscience?

I've searched various convention and Democratic sites but can't find a specific rule on it. And I'm not asking about superdelegates.

I ask because I'm curious if the Clintons are staying in the primary race until the convention, hoping to be able to arm-twist released delegates and thereby secure the party's nomination. It's a frightening thought but very Clintonian.

Thanks!