Many of you who read this site understand Microsoft's Windows testing rings and the risks involved in each one of them.
I am a big fan of not running Fast Ring builds on your production devices and I would probably stay clear of Slow Ring builds on that important hardware as well. Since the current Fast and Slow Ring builds that Windows Insiders receive are from the development branch known as Redstone 1 - aka the Windows 10 Anniversary Update - it is a good idea to stay clear of those except on pure test devices.
However, if you want to test a secondary system and get early looks at Cumulative Updates and some of the app updates for Windows 10 Current Branch (CB) then you should consider the Release Preview Ring for testing.
This ring is considered stable, so while I would not run it on a primary device that I depend upon, I am OK with running on a spare machine to get a look at updates for the Windows 10 CB release of the operating system.
Enrollment into the Release Preview Ring is just like enrolling a device into either the Fast or Slow Rings and is done in the Settings > Update & security > Windows Update > Advanced options.
In fact when you go through this process it defaults to the Release Preview Ring so you have no risk of accidentally enrolling in one of the other rings.
The gallery will walk you through the enrollment process on a Windows 10 CB (Build 10586.318) PC. The process with a Windows 10 Mobile device on CB will follow very similar steps.