Crimestop first appears when Winston is reading the "Ignorance is Strength" section of The Theory and Practice of Oligarchical Collectivism. On pages 211-212 of 1984, The Theory and Practice of Oligarchical Collectivismteaches that a party member--a member of Oceani'a ruling class--"must have private emotions and no respites from enthusiasm...the first and simplest stage in the discipline, which can be taught even to young children, is called, in Newspeak, crimestop. Crimestopmeans the faculty of stopping short, as though by instinct, at the threshold of any dangerous thought. It includes the power of not grasping analogies, of failing to perceive logical errors, of misunderstanding the arguments if they are inimical to Ingsoc, and of being bored or repelled by any ttrain of thought which is capable of leading a in a heretical direction...[it] demands a control over one's own mental process as complete as that of a contortionist over his body."
Enotes.com simplifies this concept in the following summary: "Commitment to the Party, combined with hatred of the enemy, shapes the life of every Party member, who has been taught from the earliest ages the skill of 'crimestop,' the faculty of stopping any dangerous thought. The need for flexibility in dealing with facts demands their continuous alteration made possible by 'doublethink.'"
Enotes.com also identifies when, in Part Three, Winston tries to imploy that tactic: "Winston practices 'crimestop,'a Newspeak term for the automatic process by which the mind stops any dangerous thought. In the back of Winston’s mind, however, is the recognition that the only sure thing in his life is the certainty that he will be shot from behind. What remains unclear is when this will occur."