Here's an interesting perspective on this, though the article (portion copied below) references the squat. I have always tightened my core (i.e. flexed abdominal musculature and spinal erectors, what I would consider to be valsalva, exhaling against closed glottis thus no air escaping out your mouth). I tend to self restrain my core in this maneuver, i.e. would feel very tight even without the belt. I believe the variant suggested is to force your abdomen out against the belt, using the restraint of the belt against your abdomen to stabilize your core, as opposed to a tight core even without the belt present. Subtle difference, I've been experimenting with this.
XVXhttp://articles.elitefts.com/articles/t ... ad-to-toe/
EFS Classic: Squatting from Head to Toe
By Dave Tate Published: July 26, 2010
This article was first published at t-muscle.com PUBLISHED 08-31-00
"Now that your upper back is tight you’ll need to tighten your midsection. First, expand your abdomen as much as possible. When you pull air into your body it should be into the diaphragm, not the chest. Expand you belly and push it out against your belt. This will stabilize and support the lower back and not elongate the spine. If you’re having a hard time trying to figure this out, then wear your weight belt one notch loose and push into it with your belly so it becomes tight.
Pushing your belly out goes against what many believe because they feel training this way will cause injuries to the lower back. After 30 years of box squatting Westside has had 23 lifters squat over 800 pounds, six over 900 pounds and one over a grand. Not one of these lifters or any of the others has had lower back problems.
Another aspect of this to keep in mind is the circumference of the waist line. If I suck my belly in my waist line measures 42 inches. If I pull air into my belly and push it out it measures 48 inches. The wider base the stronger the lifter. This is why lifters with a bigger waist squat more. The pyramids in Egypt are also built with a wide base and they have been standing for centuries. As the car commercials used to say, wider is better.
I learned this lesson firsthand at the 1990 Toledo Hall of Fame powerlifting competition. I’d just tried a 760 squat and got smashed with it. This was my second attempt of the day and I decided to give it another try on the third. I had some doubts because the second attempt wasn’t even close. Saying I got smashed is an understatement. The weight stapled me to the floor! I didn’t even get out of the bottom of the lift. This weight was a 20 pound personal record for which I had spent the last four months training.
I didn’t understand what the problem was or how to fix it. On the third attempt, while I was getting wrapped, Louie Simmons walked up to me and told me to get my abdominals tight. I had no idea what he was talking about at the time, but would within the next few minutes. As I got under the weight I realized Louie was the spotter behind me. (No pressure there, huh?) As I got set under the bar he told me to expand and push my belly into the belt. Now I understood what he was talking about. I was always told to flex my abs, but never to expand and push out.
As I set the bar up, I noticed that I had never felt so tight and stable. Once set, I locked in my back and began the squat. I kept my belly pushed into the belt and blasted the weight up! I had just smoked a weight that stapled me to the floor moments earlier all because I learned how to use my abdominals! In my opinion, this concept is one of the most misunderstood in the sport of powerlifting today. Many lifters don’t know how to use their core to set up a squat. Some do nothing at all while others are trying to suck their stomachs in. This is probably fine for those who strive to squat 400 pounds, but if you’re looking to squat maximal weights in the 700 to 900 range, you’d better learn how to use your core.
All the power of the lower body is transferred through your core to the barbell. If this core isn’t tight the power will “get lost” so to speak and never travel to the bar. While I don’t agree with the use of a belt for the majority of training, I do believe in the use of belts to teach a person how to use the abdominals while squatting. The belt is a training aid in competition, so you must learn how to use it to its fullest advantage."