I've heard (never seen it done, or worked for a place that did this) that they withhold a last paycheck, severance, whatever until you sign. Probably not legal but it is pretty compelling at a very shocking time.

I'm pretty sure that, in order to get severance, you will often need to sign a release form of some kind. But withholding any pay for work done or unpaid vacation is illegal.

If a company has such poor ethics that they'd think about witholding a last paycheck, what makes you think signing is going to prevent it from happening anyways?

My suspicion was that they were bluffing. Again, this is all about 3-rd hand; never happened to me or someone I know.

Even such a bluff is ethically grey, and a dark shade at that.