These are bids that show controls (aces/voids and kings/singletons) in the bid suits, usually used in slam explorations.
The first time a suit is cue bid, it shows a first round control (Ace/void). The second time a suit is
bid, a second round control (King/singleton) is shown.
If partner cue bids to show a 1st round control, and responder lacks any, responder rebids
the agreed-upon suit. If the partner continues to cue bids, s/he tends to show 2nd round controls.
Responder's subsequent bids therefore also are 2nd round controls.
S: x H: Jx D: KJxx C: KQxxx S: AKx H: AKQxxx D: x C: Axx W N E S 1H P 2C P 2H P 3H P 4C P 4H P 4S P 5C P 5D P 6H P/O