What is the WikiLeaks 'dead man's switch' (Insurance File)?

WikiLeaks 'dead man's switch'? Assange's arrest prompts speculation about possible major data dumps
https://www.rt.com/news/456441-insurance-files-assange-wikileaks/

"Anti-secrecy organization Wikileaks just released a treasure trove of files that, at least for now, you can't read."
https://www.businessinsider.com/wikileaks-insurance-file-2013-8

"Now that Julian Assange has been arrested in London after seven years in exile at the Ecuadorian embassy, many are wondering if anything will happen with the “dead man’s switch” that Assange and WikiLeaks have talked about in the past."
https://heavy.com/news/2019/04/julian-assange-dead-mans-switch-wikileaks-insurance-files/

"The files have been described as a “deadman’s switch”, or more commonly, as an insurance policy."
https://www.theweek.in/news/world/2019/04/12/After-Julian-Assange-arrest-fate-of-88GB-insurance-files-uncertain.html

"The tweet, which remains Assange's last entry as of 5pm GMT, includes a 60 character code composed of letters and digits, along with a link to M.I.A.'s Paper Planes, a song featuring the chorus "All I wanna do is (gunshot, gunshot, gunshot, gunshot) and (click, ka-ching) and take your money" and the lines "If you catch me at the border I got visas in my name" and "I've got more records than the KGB, So, uh, no funny business.""
https://sputniknews.com/viral/201801011060466020-mysterious-assange-tweet/

"the secret-spilling site has posted a mysterious encrypted file labeled "insurance.""
https://www.wired.com/2010/07/wikileaks-insurance-file/

"WikiLeaks has released a massive encrypted cache of documents as an “insurance policy”"
https://www.theregister.co.uk/2013/08/19/wikileaks_prepare/

How Secure Is Julian Assange's "Thermonuclear" Insurance File?
https://www.popsci.com/technology/article/2010-12/how-secure-julian-assanges-thermonuclear-insurance-file/

"The insurance files are available as a torrent files. Huge files.
What no one has, is the encryption key to open the insurance files."
https://twitter.com/JustinCase0912/status/1117210586356617218