Actually, in my experience with a PIP, it doesn't threaten you'll be fired today.
It's emotional blackmail. I was told, in black and white, that I sucked and I was holding everyone back. For anyone with any self-doubt or need for social approval, a statement like that will feel like a ton of bricks. But then they offer a path to redemption: sign a document, then complete these tasks, and all will be well again. Since I have a programmer's ego I feel I can code my way out of any situation. So I signed.
And I did so knowing full well that I was doing something against my interests. But oddly, at the time, that didn't matter as much to me as redeeming myself in other people's eyes.
If this ever happens to you, the thing to recognize is that at that moment, the company is no longer your friend (if it ever was) and from now on, every dealing with you is probably going to be backhanded. Loyalty and pride in accomplishment are suddenly bad traits for you to indulge in.
The best option may be to sign and slack off while you job-hunt. Or, if you are quick to realize that there is no way out, to offer to go away, in exchange for a decent amount of severance.
Either way, I was foolish to sign without getting a lawyer to examine the document first.