How does O’Brien use a previous conversation with Winston to demoralize him?

When O'Brien first goes in to Winston, Winston believes that he too has been arrested. O'Brien however uses a prior conversation to reveal hinself and explains, "The got me a long time ago...You knew this Winston. Don't deceive yourself. You did know it - You have always known it." In context, O'Brien is acknowledging that he is a Party member, and that Winston has really always been aware of it.