Did you ever, as a kid, build a LEGO set and make a mistake early on, figuring it out later and having to undo all your work until you found the error? Imagine that the mistake cost someone their life, and you really needed that life to finish the project. That’s what’s frightful about the 10 pints trap: the realization that everything up to that point has been done wrong, building on a fundamental mistake. The traps set for five players along the way only seemed to be traps of attrition — though they could be played that way, they could also be solved by everyone cooperating. As Jigsaw guessed, though, all involved were too selfish to consider the possibility.
So for the final trap, 10 pints of blood must be drawn by saw blade (natch) to fill a beaker to stop some bombs from detonating, and had the five players all survived, they could each give two with relative ease. Unfortunately, only two remained, meaning each would need to give half the blood in their body. The average adult human contains 10-12 pints; losing more than half is usually fatal.
Still, even with good reason to hate each other — Mallick (Greg Bryk) set fire to a building thought abandoned, inadvertently killing eight people, and Brit (Julie Benz) indirectly hired him — the two finally pooled their resources.
And bodily fluids.
Source From: www.slashfilm.com