InstantSend (formerly InstantX) Transactions in Dash version 12 are secured using a consensus of deterministically selected masternodes.
This set of masternodes is informally termed quorum and must be in a majority agreement, at least six out of ten, for a successful lock of the transaction inputs.
Quorums are self-selected on a per-transaction basis using a combination of three elements: Transaction Height Age, Block Height, and Masternode Funding Transaction.
When an InstantSend transaction is broadcast, the height age of the first unspent input in the transaction influences which proof of work hash to use as entropy for quorum selection. All InstantSend inputs must be at least six blocks old or the transaction will be rejected.
The height age of the first InstantSend input is then subtracted from the current blockchain height. This guarantees a new source of entropy every time a block is discovered. To prevent using transaction malleability to influence quorum selection, four is then added to the calculated block height to guarantee choosing a block generated after the input was created.
Each masternode receiving the InstantSend transaction lock request then compares the hash of its funding transaction to the hash of the block calculated above. After validating the inputs are not spent, the top ten masternodes closest to this proof of work hash broadcast their acceptance of the lock.
sorry took so longyes (double checked with Udjin) good point / ideafixed