I thought the best way to improve your bench form was to work on your bench form. Not roll your back over varying sizes of PVC pipe and do stretching exercises.
Granted, the stretching exercises for the legs might help your hips get better accustomed to the wide and tensed leg position required for power benching, but the instructor doesn't get what proper bench form is and does.
PROPER bench form turns your body into a coiled spring; your back arch isn't created by pipe rolling but the result of you winching yourself into position.
Here's how I have described it previously:
...the setup is the most important part of the lift, and takes the most time to get right.
You need to get under the bar and slide up the bench until your belly button is within a couple of inches from being aligned with the bar. Keep your feet flat on the floor, your knees bent at a comfortable 90 degree angle.
Now grab the bar with both hands and - with feet planted firmly - swing/slide yourself down the bench until your eyes are in line with the bar (or at whatever alignment you find most comfortable). Now, if your feet have stayed where they were, you will now have a tight arch in your back, and your legs will likewise feel coiled and tense.
Lower the bar with your elbows as close to your sides as possible, and bring the bar down closer to your midsection/solar plexus.
While this guy says he has worked with Olympic athletes... somehow I feel he's missed the point of back arch in benching.