我查阅了原文的出处，完整的句子是这样的：

Since adults know how to place the key in the slot, which is the ultimate purpose of approaching the slot, and since the child is neither old enough nor clever enough to realize the desired action on his own, what possible gain is achieved by having his struggle? 

【翻译】由于大人们知道如何将钥匙放到钥匙槽中，这样做是接近钥匙槽的最终目的，又由于小孩子既不够成熟也不够聪明，自己无法完成这个要求的动作，那么，他的努力又能得到什么收获呢？

【分析】我认为，which 既不是指代整个主句，也不是指代某一个单词（the key或the slot），而是指代整个how to place the key in the slot（= the way to place the key in the slot 将钥匙放到钥匙槽中的方法）。所以，把which 翻译成“这样做”。

which引导非限制性定语从句时，有一个特殊的用法，就是：它除了指代主句中的某个名词或者整个主句的内容外，还可以指代主句中的某个名词词组（how to do sth. 也属于名词性词组）、形容词或形容词短语。例如：

The man was unable to earn enough money, which allowed him to support his family.

那人无法赚到足够的钱，只有足够的钱才能让他得以养活家人。

which 指代enough money，不是单个的money。更不是整个主句：整个主句的意思是“无法赚到足够的钱”，既然赚不到钱，怎么养活家人？

又如：

1. He was not sick, which some of the other passengers were.

他没有生病，但有些其他旅客却病了。

（which指代形容词sick）

2. She is always careless, which we should not be.

她总是马虎大意，我们可不应该这样。

（which指代形容词careless）

3. The two policemen were completely trusted, which in fact they were.

那两个警察完全受到信任，事实上，也真是如此。

（which指代形容词短语completely trusted）

4. She was very patient towards the children, which her husband seldom was.  

她对孩子们很耐心，她丈夫却很少这样。 

（which指代形容词短语patient towards the children）