GMC 2010 Sierra Denali Owner's Manual page 60

Q: What is wrong with this?
A: The belt is twisted across the body.
2-24
WARNING:
You can be seriously injured by a twisted belt. In a
crash, you would not have the full width of the belt
to spread impact forces. If a belt is twisted, make
it straight so it can work properly, or ask your
dealer/retailer to fix it.