BIP 101 was implemented in Bitcoin XT on August 6, 2015 and released on August 15. If 75% of the last 1000 blocks were observed to have particular version bits set, it would have ceased enforcing the 1 MB block size limit and also have added a few new rules. However, this threshold was never achieved and no new XT blocks are being created today.
In January 2016, BIP 101 was removed from Bitcoin XT and replaced with the one-time block size increase to 2 MB present in Bitcoin Classic. In the year following this change, adoption of Bitcoin XT decreased dramatically, with fewer than 30 nodes remaining by January 2017. Later attempts by other codebases to increase the block size to 2 MB including Bitcoin Classic and SegWit2x have also failed.
Miners that side with Bitcoin XT will produce blocks with a new version number. This indicates to the rest of the network that they support XT. When 75% of the last 1000 blocks are new-version blocks, these miners will automatically abandon Bitcoin and begin mining on a new Bitcoin XT blockchain. This will begin after a waiting period of two weeks in hopes the economy in this time may force anyone who hasn't switched yet to do so.
Users and miners running full Bitcoin nodes will reject the XT blockchain starting with the first block that is larger than one megabyte in size, and thus be unaffected provided it fails to achieve economic consensus.
Users and merchants
If insufficient mining hash power runs XT to reach supermajority then nothing will happen. If enough does, XT users will follow a new blockchain and cease to be using and trading bitcoins.
Later in January 2016, frustrated of his proposal being massively outvoted, Mike Hearn made a media stunt declaring on various US national and international press agencies that "Bitcoin has failed". Max Keiser quoted Bram Cohen  in describing Hearn's move as "whiny rage quitting"  and other members of the community pointed to an extensive account of manipulations http://gettopical.com/bitcoin/e289a9a5189008d994e80666e9038089 (unfortunately today many twitter screenshots are broken).
On June 10, 2014 Mike Hearn published a Bitcoin Improvement Proposal (BIP 64), calling for the addition of "a small P2P protocol extension that performs UTXO lookups given a set of outpoints." On December 27, 2014 Hearn released version 0.10 of the client, with the BIP 64 changes.
On June 22, 2015, Gavin Andresen published BIP 101 calling for an increase in the maximum block size. The changes would activate a fork allowing eight MB blocks (doubling in size every two years) once 75% of a stretch of 1,000 mined blocks is achieved after the beginning of 2016. The new maximum transaction rate under XT would have been 24 transactions per second.
The August 2015 release of XT received widespread media coverage. The Guardian wrote that "bitcoin is facing civil war".
Wired wrote that "Bitcoin XT exposes the extremely social — extremely democratic — underpinnings of the open source idea, an approach that makes open source so much more powerful than technology controlled by any one person or organization." Hashcash developer Adam Back was critical of the 75% activation threshold being too low and that some of the changes were insecure.
Bitcoin Cash support
On August 25, 2017, Bitcoin XT published Release G, which was a Bitcoin Cash client by default. Subsequently, Release H was published, which supported the November 2017 Bitcoin Cash protocol upgrade, followed by Release I, which supported the May 2018 Bitcoin Cash protocol upgrade.
- organofcorti (27 August 2015). "BIP101 implementation flaws". The Neighbourhood Pool Watch. http://organofcorti.blogspot.com/2015/08/bip101-implementation-flaws.html. Retrieved 27 August 2015.
- Hearn, Mike (27 August 2015). "An XT FAQ". Medium. https://medium.com/@octskyward/an-xt-faq-38e78aa32ff0. Retrieved 27 August 2015.
- Mike Hearn. "Why is Bitcoin forking? — Faith and future". Medium. https://medium.com/@octskyward/why-is-bitcoin-forking-d647312d22c1.
- "Bitcoin XT Nodes Summary". coin.dance. https://coin.dance/nodes/xt. Retrieved 7 January 2017.
- Higgins, Stan (November 17, 2017). "No Fork, No Fire: Segwit2x Nodes Stall Running Abandoned Bitcoin Code". CoinDesk. https://www.coindesk.com/no-fork-no-fire-segwit2x-nodes-stall-running-abandoned-bitcoin-code/. Retrieved November 29, 2017.
- Wilson, Fred (17 August 2015). "The Bitcoin XT Fork". AVC. http://avc.com/2015/08/the-bitcoin-xt-fork/. Retrieved 28 August 2015.
- Whiny Ragequitting https://medium.com/@bramcohen/whiny-ragequitting-cab164b1e88
- Mike Hearn's move from Bitcoin to Banks - Stacy Herbert, Max Keiser & Simon Dixon discuss https://www.youtube.com/watch?v=ubqjei3Jd-U
- "bips/bip-0064.mediawiki at master · bitcoin/bips · GitHub". GitHub. https://github.com/bitcoin/bips/blob/master/bip-0064.mediawiki.
- "bitcoinxt/bitcoinxt". https://github.com/bitcoinxt/bitcoinxt/releases/tag/v0.10.
- "bips/bip-0101.mediawiki at master · bitcoin/bips · GitHub". GitHub. https://github.com/bitcoin/bips/blob/master/bip-0101.mediawiki.
- Tim Hornyak (21 August 2015). "Bitcoin XT debate overshadowing growth opportunities". PC World (IDG). http://www.pcworld.com/article/2974339/bitcoin-xt-debate-overshadowing-growth-opportunities.html. Retrieved 7 January 2017.
- "Implement hard fork to allow bigger blocks · bitcoinxt/bitcoinxt@946e3ba". https://github.com/bitcoinxt/bitcoinxt/commit/946e3ba8c7806a66c2b834d3817ff0c986c0811b.
- "bitcoinxt/bitcoinxt". https://github.com/bitcoinxt/bitcoinxt/releases/tag/v0.11A.
- "2MB block size bump by dgenr8 · Pull Request #117 · bitcoinxt/bitcoinxt". https://github.com/bitcoinxt/bitcoinxt/pull/117.
- Alex Hern (17 August 2015). "Bitcoin's forked: chief scientist launches alternative proposal for the currency". the Guardian. https://www.theguardian.com/technology/2015/aug/17/bitcoin-xt-alternative-cryptocurrency-chief-scientist. Retrieved 20 August 2015.
- Cade Metz (19 August 2015). "The Bitcoin Schism Shows the Genius of Open Source". WIRED. https://www.wired.com/2015/08/bitcoin-schism-shows-genius-open-source/.
- Everett Rosenfeld (20 August 2015). "Bitcoin splits: Will it break, or be better than ever?". CNBC. https://www.cnbc.com/2015/08/20/bitcoin-splits-will-it-break-or-be-better-than-ever.html. Retrieved 5 January 2017.
- "Bitcoin XT Releases". https://github.com/bitcoinxt/bitcoinxt/releases. Retrieved 17 June 2018.