Harry became aware that something was still struggling in his right hand, the knuckles of which he had bruised against Malfoy’s jaw.

【翻译】哈里意识到他的右手像有什么异物仍在隐隐作痛，原来他在马尔福的下巴上擦伤了右手的指关节。

which引导非限制性定语从句，which 指代his right hand，of which作定语修饰the knuckles（手指关节）。本句可以改写为：

Harry became aware that something was still struggling in his right hand, of which the knuckles he had bruised against Malfoy’s jaw.

直接使用whose 表示“所属关系”：

Harry became aware that something was still struggling in his right hand, whose knuckles he had bruised against Malfoy’s jaw.

我再举两个类似的句子：

【非限制性定语从句】

I want to help those children, whose parents died in the earthquake.

I want to help those children, of whom the parents died in the earthquake.

I want to help those children, the parents of whom died in the earthquake.

【限制性定语从句】

We must repair the desks whose legs are broken.

We must repair the desks of which the legs are broken.

We must repair the desks the legs of which are broken.