Here's a couple videos from the post that give a close up look at hip rotation from the side and front views:
Hip Rotation Side View
Hip Rotation Front View
The article itself describes what's going on in the videos based on EMG research from both golf and baseball, and has some thoughts on how you can work on your hip action in the weight room as well as the batting cage.
One line from the article warrants repeating here:
Here’s the key point: good hip rotation has an element of hip extension!
Give the article a read and spend some time at Eric's website or follow him on twitter. He's doing a lot of good things in the world of baseball training.