Anti-submarining ramps don't have anything to do with submarines, I'm afraid.
During a car crash, when a person slides underneath the seatbelt, that is called submarining. The seatbelt slides up onto the stomach, and can cause injury.
An anti-submarining ramp is a ramp built into the seat cushion, underneath the foam padding, that helps to prevent the person from sliding forward on the seat underneath the seatbelt. This keeps the seatbelt in the proper position on the person's pelvis, and helps to prevent injuries to the stomach.
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