From the Microsoft post announcing the update (with my emphasis on the omst important fixes):
Today we are releasing another Cumulative Update for Windows 10 Mobile Insider Preview Build 10586 to Windows Insiders in the Slow and Release Preview rings: Build 10586.122. This Cumulative Update continues to address feedback we received on Windows 10 Mobile from devices such as the Lumia 950, 950 XL, 550, and 650 as well as the upgrade experience from Windows Phone 8.1. In some cases, improvements and fixes in this Cumulative Update will only be seen when upgrading from Windows Phone 8.1. We are also adding support for 4 additional devices on the Current Branch for Insiders receiving today’s update: MCJ Madosma Q501, BLU Win HD W510U, BLU Win HD LTE X150Q, and ALCATEL ONETOUCH Fierce XL.
Windows Insiders with devices on the Fast ring which are not yet receiving updates from the Development Branch would need to temporarily switch to the Slow or Release Preview rings if they want to receive this build.
Here are the fixes included in this release:
- Improved Kanji input experience while using Continuum.
- Improved the speed in which thumbnails appear for portrait videos.
- Improved Internet Sharing on select mobile network configurations.
- Fixed an issue in Microsoft Edge that prevented Word Flow input on some sites.
- Fixed an issue that previously prevented the option to “Close all Tabs” in Microsoft Edge from closing all open tabs.
- Improved IMS registration on select mobile network configurations.
- Fixed an issue where adding your MSA during OOBE would fail and prevent the account from being re-added after the failure.
- Improved the email sync experiences when connecting to Wi-Fi networks with a captive portal.
- Improved data connection profiles for dual SIM devices.
- Updated background processing of multiple changes to improve battery life of devices.
- Improved quality of video record to SD memory cards.
- Improved video recording audio in some cases.
- Improved overall OS reliability, including reliability of the Start screen, app updates, and the Windows Feedback app
- Improved Alarm reliability.
And here's the pitch to anyone willing to help test the overall upgrade process:
If you’re a Windows Insider who is willing to help test the upgrade path that most customers will be taking from Windows Phone 8.1 – you can use the Windows Device Recovery Tool to go back to Windows Phone 8.1 and then download the Windows Insider app from the Store and jump into either the Slow or Release Preview rings to pick up Build 10586.122 for your phone. Make sure to send feedback on any issues you encounter through the upgrade process or after the upgrade completes. We appreciate Insiders helping us test out the upgrade from Windows Phone 8.1 to help ensure our customers have a great experience.
The very fact that .122 is being pushed out to both Slow and Release Preview rings at the same time is indicative of how highly Microsoft thinks of it. And of how urgent the need is to start getting the upgrades out, of course. Though, allowing for time for testers to feed back, you've got to think that we're even now still a week or two away, at minumum?
PS. The various rings are still a little confusing, IMHO. I thought the 'Slow ring' was for the more stable releases on the new Redstone (14267) branch? Plus the quoted message refers to Insiders on the Fast ring who haven't yet received the Redstone upgrade - maybe this just applies to the WP8-native devices with their Insider setting still set to 'Fast'? I'm sure all this makes sense to someone in Microsoft, with a clear white board covered in lines and blobs, but I'm fast losing track, if I'm honest!