Pinch gripping involves squeezing the extended fingers towards the thumb without flexing individual digits, such as you'd experience holding a textbook.
Most strength athletes use plate pinches (holding two 25 or 45 pound plates together), but there are alternatives. For example, using two dumbbells allows you to have access to a greater range of weights. That makes it much easier to progress in smaller increments.
If a trainer were to come to us and say he had a method of ab training that would reduce the waist size of anyone in three weeks, we'd ask him what he was selling and then kick him out the door. Luckily, Don Alessi was a fast talker and before we could introduce him to the sidewalk, he'd convinced us that he had some pretty nifty ideas.