How does a contested convention work?

[Bob41213]

I heard so much about the Republican contested convention originally, but now I'd be curious about a Democratic one.

Are all pledged delegates only bound to vote for a person on the first vote?

Has Bernie carefully chosen delegates such that he can trust them reasonably well? I remember hearing so much about how Trump didn't know how to pick delegates and ended up getting Cruz people in there, etc.

I'm not sure what I'm expecting but I think it's more and more likely that Hillary may be in trouble come convention time. Not exactly sure how things will play out either.

[woodsprite]

1. I hope it works out as well for Bernie as it did for FDR and the US in 1932. n/t