TinyUmbrella is a very recognizable name among jailbreakers, and before now, offered those partial to Cydia tweaks a way of installing old iOS software even if Apple had ceased to sign it. Particularly useful to those who might have updated in haste, TinyUmbrella offered a get-out-of-jail-free card to those who'd saved their SHSH blobs for a particular iOS version, spoofing an Apple server which could then downgrade without too much trouble. Having been out of the spotlight for a while, TinyUmbrella is currently in the process of being revived, and now, has been given a healthy update that supports SHSH fetch on A7 and A8 devices.
Developer Semaphore only pushed the last update just over one week ago, and where it could save blobs for software versions that Apple was signing, today's beta will also fetch SHSH blobs for whatever you are currently running. So if you've sat patiently on iOS 8.1.2, you can now use TinyUmbrella to save the SHSH blobs for your device, meaning that, in future, you'll be able to revert back to iOS 8.1.2 if you wish.
There are one or two conditions, though. Firstly, your device will need to include an A7 or A8 processor. According to the release notes on the TinyUmbrella blog, this includes the iPhone 5s, iPad Air, iPad mini 2 and iPad mini 3, all of which are A7, as well as the iPhone 6, iPhone 6 Plus and iPad Air 2, powered by the A8 chip.
As well as only supporting A7 and A8 at this stage (other devices will have to wait for iH8Sn0w to update his iFaith tool), you will also need to be jailbroken in order to use TinyUmbrella to fetch your blobs. Once you have jailbroken, it will also be necessary to have AFC2 installed, and you can do this by searching Apple File Conduit "2" in Cydia.
To clarify, you cannot yet downgrade using TinyUmbrella. The scraping of SHSH blobs means that, at some point down the line, a downgrade will be possible, but even though you can't revert to old software just yet, this doesn't mean you shouldn't get into the habit of saving the blobs.
The more SHSH blobs you have stored for different iOS versions, the broader your choice when downgrading does become possible, and since Semaphore appears to be on top of his game right now, it may not be long before it happens.
For now, TinyUmbrella beta 8.2.0.48 gives you everything you need to preserve iOS 8.1.2 if you're still running it. We would not recommend updating to iOS 8.2 if you want to jailbreak, though, because it cannot yet be jailbroken, and since the most recent public build carries its fair share of bugs, it's probably best if you stick with iOS 8.1.2 regardless.
You can download TinyUmbrella from the official site.
