"Now, I must warn you that the most stringent Anti-Cheating Charms have been applied to your examination papers. Auto-Answer Quills are banned from the examination hall, as are Remembralls, Detachable Cribbing Cuffs, and Self-Correcting Ink."
Banned during O.W.L. and N.E.W.T. Examinations, this item is used for cheating.
The Detachable Cribbing Cuff is presumably one of the items used for cheating, banned during O.W.L. examinations when "...a flourishing black-market trade in aids to concentration, mental agility, and wakefulness" erupted prior to fifth and seventh years' examinations. (OP31)