不了解to which是什么使用方法

They have pleasant mouths turned up at the corners, to which the great Master of Marionettes has fixed the strings and he holds them in his nimblest fingers to twitch them at the slightest jest.

我不了解to which 在这里是什么使用方法，为何译文会在这里翻译收获像木偶大师?

to which与fixed the strings构成fix something to something短语，意思为“将某物固定于某物”。这里的which引导定语从句，先行词可能是corners。