There's a reason they make denim powerlifting gear - same lines as canvas. It provides support and that let's you lift more. It's a pretty solid fabric and popular in bench shirts. Granted you aren't getting outrageous benefits and they aren't ultra tight but anyone that's squatted in jeans or jean shorts knows it makes a difference and they seem stronger and more stable (not every area has to be tight but as you move the fabric stretches and it is held in place by your body and at your waist and supported by the seams). And this is why you made the post, because you've discovered the properties of demin even though that wasn't the purpose and you realize it lets you handle more weight and have more stability. In typical training, I'd advise avoiding supportive gear for the most part. In addition, those shorts are not made for heavy loads, this is not lifting gear, and if you come to rely on them and they blow up on a heavy attempt - that can destabalize your lift which is generally undesirable and can lead to injury.