The seat belt restrains the passenger from moving forward during sudden braking.

restrains 是动词 restrain 的第三人称单数形式。restrain 由前缀 re-（表示"加强"）和词根 strain（"拉紧"）组成，意思是"限制、约束、阻止"，指用外力控制某物或某人的移动或行动。