When a company says that its clamp can extert X amount of force, what does
that mean? Before the clamp will snap? That a user will not be able to
tighten it any further?

Specifically I am interested in the specs on the "Quick-Grip One-Handed Bar
Clamp / Spreader" shown he
http://www.coastaltool.com/cgi-bin/w..._bar_clamp.htm

It says that the clamp "Exerts up to 250 lbs of clamping pressure". I've
used this clamp and tightened it as best I could by hand, squeezing the
trigger. Assuming I have a "normal" strength grip, would that mean that at
that point the clamp is exerting 250 pounds? What might the multiplier be?
What if I used another clamp to squeeze the trigger of the first clamp?
TIA. -- Igor