You can be seriously hurt if your shoulder belt is; too loose. In a crash, you would move forward too; much, which could increase injury. The shoulder- Page 45

1-33

The lap part of the belt should be worn low and snug on
the hips, just touching the thighs. In a crash, this applies
force to the strong pelvic bones. And you’d be less likely
to slide under the lap belt. If you slid under it, the belt
would apply force at your abdomen. This could cause
serious or even fatal injuries. The shoulder belt should 
go over the shoulder and across the chest. These parts 
of the body are best able to take belt restraining forces.

The safety belt locks if there’s a sudden stop or a crash,
or if you pull the belt very quickly out of the retractor.

CAUTION:

You can be seriously hurt if your shoulder belt is
too loose. In a crash, you would move forward too
much, which could increase injury. The shoulder
belt should fit against your body.

To unlatch the belt, just push the button on the buckle.