Swann’s friends know him as a man who looks gruff and uneducated but also happens to be a brilliant mechanic with knowledge rivaling that of the sector’s top terran engineers.

1. rival：及物动词，意为“比得上；与......相媲美；与......竞争”。

2. rivaling that of the sector’s top terran engineers：现在分词短语作定语，修饰knowledge。

3. that指代上文的knowledge。

a brilliant mechanic with knowledge rivaling that of the sector’s top terran engineers. 

等于：

a brilliant mechanic with knowledge which rivals the knowledge of the sector’s top terran engineers.