其中的 be knocked 为被动语态，from under sb 属于以介词短语为介词宾语。

关于 from under...的理解我们可以看看以下几句：

He came out from the bed. 他从床那儿出来。（意思不明确：可能是从床旁边或后面等）

He came out from under the bed. 他从床下面出来。（意思明确）

关于 from under 的更多例句：

They crawled from under the table. 他们从桌底爬过来。

A mouse ran out from under the wall. 有一只老鼠从墙底下跑出来了。

He took a box from under the counter. 他从柜台底下取出一个盒子。

类似例句，如 from behind：

The sun emerged from behind the clouds.  太阳从云层后面露了出来。

The dog’s head peeked out from behind the tree.  树背后露出了那只狗的脑袋。

She peeked at the audience from behind the curtain. 她从帷幕后面窥视了一下观众。