Bridge - Conventions Bridge - blackwood turbo.
It is a slam zone convention, which says, that a cue-bid bid of 4N means having an even number of aces (z 4), and exceeding this bid, without her, it speaks of an odd number of aces.
This convention can be helpful, when a minor color is agreed, and the responder to Blackwood has only one ace.
There is also a variation, that bidding with 4NT indicates that 0 the 3 Ashy (z 5), while bypassing on 1 the 4. Z 5 asami, on the other hand, you have to bravely steer your partner to bid 4NT.