Redouble is to counter opponent’s double. It is sparsely used but has specific meaning under special situations:

• Whenever opening bid is doubled, responder can redouble to show 10+ Points invitational hand. Responder doesn’t have 3 card support for opener. Use Truscott 2NT, instead to show both 3+ card support to partner’s Major and also Invitational hand.

• Redouble when you intended to show Support Double to responder for 3 card support but the opponent interferes with a double.
• Redouble by responder on 1NT opening and doubled by over caller, shows 5+ cards long minor. BBO robots use this convention.
• Rebouble by opener as 2nd bid to show value i.e. opening hand with invitational values. This situation will occur when next two seats pass after opening bid (including responder) and advancer doubles.
• ‘Redouble, technically, is a counter to penalty double, but is rarely used.