Currently InstantSend (previously known as InstantX) on default settings provide 5 instant confirmations from masternodes, locking the input amounts of that transaction to protect it against double spending. The 6th confirmation comes from proof of work by miners and that will provide transaction finality in wallets and stores the transaction on the blockchain.

Link : https://docs.dash.org/en/latest/introduction/features.html#instantsend
White Paper : https://github.com/dashpay/docs/blob/master/binary/Dash%20Whitepaper%20-%20Transaction%20Locking%20and%20Masternode%20Consensus.pdf

About your question with regards to if, when and how this works in case of blockchain reorganizations i'm not sure.
I suspect instantx confirmations get stored in the mempool, but maybe someone with more knowledge can provide a more definitive answer...