When Winston and Julia go to O'Brien's apartment, what does Winston find particularly remarkable about the telescreen?

In Part 2 Chapter 8 of 1984, Winston and Julia go to O’Brien’s apartment. Like other citizens of Oceania, O’Brien has a telescreen in his apartment, which monitors his actions, but with one key difference: O’Brien has a switch to turn the telescreen off. This is a revelation to Winston and Julia who have never seen anybody turn one off before. But, as O’Brien explains, all inner party members have this "privilege". It is not advised, though, to turn the telescreen off for more than half an hour, as it may arouse some unwanted suspicion. Nevertheless, this is an important moment in the book. By turning off the telescreen, O’Brien convinces Winston that he is part of the Brotherhood, a secret organisation which seeks to destroy the party. This lures Winston into a sense of security which we know is wholly false and misplaced. O’Brien is, in fact, a member of the Thought Police and Winston has just committed thoughtcrime right under his nose.