Nobody's saying how they managed to do it, but yesterday the folks at OSBetaArchive discovered a copy of Windows 10 build 10074 on Microsoft's official Windows Update site. The files are in ESD format, which is readily convertible to ISO, which in turn can be used to perform either clean installs or upgrade installs from previous versions of Windows.
Within an hour, the ESD files had been converted to ISOs, and OSBeta posted the ISOs on Google Drive. Rapidly exceeding its download capacity, the files were then posted to Kim Dotcom's Mega site in New Zealand. And from there, they've become available in every far-flung corner of the world.
Most people figure that Microsoft will officially release Windows 10 build 10074 today at the Build conference. (By the time you read this, it may well be an accomplished fact.) At this point, build 10074 does not appear if you check for updates in earlier versions of Windows 10, but once the bits reach official release status, the more than 1.5 million Windows Insiders who have signed up will get the build.
Windows Insider Preview as it's now called (presumably that's a step up from the old Technical Preview) is, in my experience, considerably more stable than its predecessor, build 10061. Look for minor improvements in the Start menu and Cortana's Search box, a slightly improved tapping field in tablet mode, and a few changes in Settings. People still doesn't work. The Photo app still doesn't have Album view. And we don't yet have a clue about how updating will be handled in the final version.
Details on the improvements, and the current state of Windows 10, are in Where Windows 10 stands right now.
Build's about to start. May the odds be ever in your favor.