The rider gently turned the rowel on his spur to encourage the horse to pick up speed.

rowel 是名词，指马刺上的"小齿轮"或"星形轮"。这个词来自古法语 roelle（小轮子），是马刺末端可旋转的带齿部分，转动时会轻触马腹以发出指令。