InstantSend transactions are optional and require the Masternode network. A quorum of 10 Masternodes is selected using the hash of the last block. Because the block time is fixed at 2,5 minutes the quorum is also replaced every 2,5 minutes. Each InstantSend transaction gets checked by those Masternodes. After 5 Masternodes have confirmed the transaction the Coins used are locked. This information is shared with the whole network which makes double-spend, a thread many cryptocurrencies are facing, impossible. The process is completed in under 1 second.
The recipient of the InstantSend transaction can be certain that he is going to receive the amount that was send to him. The transaction is completed after it’s written into the next block of the Blockchain. After this has happened the transferred Dash can be used again.