Signing such a contract under duress makes it invalid. I had my lawyer significant other look into this a while back. For example if you show up for work at a new job and they put a training contract infront of you, that was not previously disclosed, you are considered under duress since you've allready quit your last one and you really have no choice but to sign. This makes the agreement null and void. I havent spoken to her about this specifically but it seems to me "sign this or loose your job" certainly seems like your signing something under duress.