It's a toss-up between keeping the elbow in the slot and getting good extension later in the swing. He has more tips on his own discussion board . To return to home page, click on open grass. The bent elbow brought near the side of the body, is sort of a measure of how far you want your hands to be from the rotation of the body. When the elbow extends out before it comes down into the side - it's called casting.
Casting reduces power because the hands are farther from the center of rotation.
During the last stages of rotation, it starts to extend in a 'punch' manner but the punch does not reduce the need for aggressive rotation of the body. The 'punch' occurs after much of the rotation but there is a period where both are working together. It's very much like throwing a rock with a sling. Rotation gets it going early, but extension allows the hands to finally be flung forward so the bat barrel can collide in a direct path to the ball, without slashing across the ball. On an inside pitch however , the hands should extend later to bring the barrel around more before the arms are released to full extension.