I assume we're talking about something like the early 80's Cannondale handlebar bag?



That photo, courtesy Google, doesn't show it very well; but I believe the bottom of the bag had an attachment for an adjustable nylon strap, and the nylon was attached to a bungee at the bottom.

As my high school Latin teacher used to say, "it seemed like a good idea at the time." But it was, in fact, a terrible design. The weight of the handlebar bag, and its contents, was held by the steel wire doodah that went around the bottom of the stem and the top of the handlebar. Go over a bump, and it worked like a trebuchet and tended to launch the bag, or its contents, skyward. The straps were intended to prevent this from happening, by effectively adding weight to the bag, increasing downward pressure on the wire support. Tension on the straps decreased the propensity to eject the contents of the bag, but also reduced the carrying capacity of the bag.