From the Manufacturer
The IRWIN 4″ C-Clamp 100 Series has many improvements over the competitions. The handle is bigger which makes it 25% easier to apply torque. This feature will reduce hand fatigue and make it easier to apply force. The double threaded screw allows you to create 50% more pressure than the nearest competitor. This will reduce hand fatigue and make it easier to apply force. The double threaded screw will give you 50% less screw deflection which will save you time by reducing drift.
Improved screw thread tightens in half the time. Larger handle for easier tightening.