SegWit2x (do not confuse with SegWit), also called B2X, was an attempt made by CEOs and owners of several Bitcoin businesses to introduce a change in Bitcoin with following goals:
- increase block size limit to be 2 times bigger, despite block size being already increased by over 50% (for users of SegWit-enabled wallets) done just 3 months earlier by SegWit update.
- remove all other Bitcoin developers from their position
- instead install Jeff Garzik as the new de-facto developer (and possibly others)
It failed to gain consensus among community:
- over 80% of miners by hashare (though they were just signaling, and this has no cryptographic or economical meaning or repercussions)
- just below 20% of users by economy (BitFinex futures market as BT2 token) (representing actual trades that were executed for money)
- around 0% of Bitcoin developers (Bitcoin Core)
Attack or upgrade
It was created in a way that would trick users into unknowingly switching to B2X from BTC, due to:
- no replay protection
- simple SPV wallets without special tools to detect the problem would accept B2X blocks as BTC blocks
- same address formats
- code changes that allowed B2X nodes to pretend to be BTC nodes in order to connect and use P2P peers from BTC network
due to this qualities, many users considered B2X and attack on BTC network.
B2X was marketed by it's creators as an "upgrade".
Project was canceled, or at least postponed by it's leaders, as announced in mailing list message: https://lists.linuxfoundation.org/pipermail/bitcoin-segwit2x/2017-November/000685.html
This resulted in the price of B2X futures (called "BT2" on BitFinex) to withing minutes drop down from around 12% to 8%, and in following hours to 1% of Bitcoin (BTC, called "BT1" as future).