GMC Yukon Owner's Manual page 81

Q: What is wrong with this?
A: The shoulder belt is worn under the arm. It should
be worn over the shoulder at all times.
WARNING:
You can be seriously injured if you wear the
shoulder belt under your arm. In a crash, your
body would move too far forward, which would
increase the chance of head and neck injury.
Also, the belt would apply too much force to the
ribs, which are not as strong as shoulder bones.
You could also severely injure internal organs like
your liver or spleen. The shoulder belt should go
over the shoulder and across the chest.
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