There's actually two versions of this, and they behave quite differently. In one version, the sparklers have an additive effect. Basically the bundle gets very hot and very bright, but it still acts like a sparkler, just a very big one.
The other version acts like an actual bomb. You take forty to sixty sparklers and you tightly wrap them together. Duct tape works great here. You then take a single sparkler and insert it into the tip of the bundle. This acts as your fuse. When you light the fuse it will act like a normal sparkler until the flame hits the tip of the bundle. Then the whole thing explodes. It's not very bright, but it's really loud and the blast is pretty strong. Last 4th Of July I cored out a watermelon and I dropped one of these in the middle. Green chunks of the watermelon flew as far as 35 - 40 feet. Next 4th I want to put one of these into a thawed turkey.
From experience, the more sparklers in your bundle, the bigger the blast. I didn't have a chance to find out where this starts to drop off though.
WARNING: These are on the wrong side of legal in most states. They're louder than any typical fireworks, so if you decide to light them in a residential area expect someone to call the bomb squad. If you get arrested or you get killed it's not my problem.
As far as the existence of two versions of the sparkler bomb, the difference is in the sparklers themselves. Only certain color sparklers from certain brands will explode.