Due to the neck pain, he could only twist his upper body slowly to look at the person behind him.

twist 是动词，意思是"扭转、转动"，指身体或物体沿轴线旋转的动作。在这里描述的是因为颈部疼痛而无法自由转动头部，只能通过缓慢转动上半身来改变视线方向。