'To the extent that' can signify change, consequence, and/or a connection. "He has become involved in his job to the extent that he has lost his social life" "Business has gotten so bad, to the extent that we will have to close some of our stores" To make things easier you could take 'to the extent' out of the above sentences. This is very common, and acceptable. He has become so involved in his job that he has lost his social life. Business has gotten so bad, we will have to close some of our stores.

In that phrase, that = what = which.