SHSH files are kind of unique serial codes, and IOS files together, that match your apple tv chip identification numbers ECID. And match the version of IOS that is running on your machine. Without the proper one (as verified on the apple.com web server) you can never do a RESTORE to your device, with an older version of the IOS (apple.com will block you with their servers).
You can only RESTORE to the latest version of software as supplied by apple.com. But the latest version might not be JAILBROKEN so its a deadend. You need the older generic version of IOS, that gets unique code from what you saved earlier with TINYUMBRELLA.
There are generic IOS files being archived right now (to numerous locations), that are labeled APPLETV4.3.2.shsh as example. Not that exact name, but similar out there.
When you want to restore version 4.2.1 of the IOS you find the generic *.shsh file, and you set up a local server application (built into TINYUMBRELLA) to reference your saved file. Then the RESTORE will work.
Hope this makes sense, and that someone can add to the explanation.