I hate it when he is lazy: What does 'it' refer to?

Dear teachers,

Please read:

I hate it when he is lazy.

What does “it” refer to?
This structure is strange for me. Do you know the rule?


This is just an introductory-it in the middle of a sentence. It serves the same purpose: to postpone another sentence element (here, when he is lazy) to a position of greater emphasis.