InstantX was where you presigned (on the block chain) your UTXO to a masternode so you can spend it instantly in the future with that masternode as the designated confirmer.
That is not how it works. Masternodes would lock outputs based on a signed request. No other masternodes would (unless hostile) approve a conflicting lock. There doesn't seem to be any real mechanism for enforcement other than assuming masternodes play nice. (There is no risk to collateral for example.)
In the event that masternodes do create a conflicting locks, then PoW blocks will resolve the conflict. I don't really understand how a merchant is supposed to rely on this, since a conflicting lock can be discovered after the merchant has accepted the supposedly "confirmed by IX" payment.
I guess it is intended for low value casual payments like buying coffee. No exchanges accept it afaik.