z指代When deeply absorbed in work
which可指代前面整句话的内容,这是语法规则.When deeply absorbed in work这一句好像少了成分，为什么可以这样省略=When he is deeply absorbed in work
英语中，过去分词作状语时，有时为了使条件、时间等一目了然，可以在分词前加when/while/if/unless等，往往可转换成一个从句，由于过去分词表被动，所以，从句为被动语态。
祝你进步！应该＝When he was deeply absorbed in work,
因为which_ he often was是过去式，对吧when he is/ was...均可