Unless, of course, you are also wearing a bunch of tools or other supplies in the pack. In which case it becomes a pivot-point for causing your spine to get levered/wrenched really hard. This is one reason why I don't wear a fanny-pack when racing dirt bikes in hare scrambles or enduros. Things that are more likely to put you in a chair for the rest of your life = bad. Things like a back brace that make you less likely = good.

Sure, I could always just quit racing, but what is the fun of that?