The Zercher squat will build your upper back and anterior core as well as isometrically taxing your biceps. You can do these with or without a box, off pins, or performed as carries.
To do the squat, the bar will sit in the crook of your elbows. It'll force you to use your anterior core and posterior chain.
Set up in squat stance and then pull the bar in close to your body. Keep everything tight and engaged and explode up. Lower it back down on the pins. You can do this as a box squat too.
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.