Many people seem to have some questions regarding to Downgrading. Let's see if we can answer those!
First of all, let's start by explaining what it is and why you would use this technique.
As we all know (or maybe not), a big part of Homebrew was made possible due to a Bug in the Wii's security code called the Trucha bug (Sometimes called FakeSign Bug). This bug allowed us to run code that does not have an official signature from Nintendo (Hence the name FakeSign, Fake Signature) on the Wii.
Nintendo then found out about this annoying bug and Patched it in several System menu updates. The first update to try this was update 3.3 (also known as the October 23th update), which blocked out the first Twilight Hack. The next Update, 3.4, did a better job and blocked out the Twilight hack for good.
Anyway, the Twilight hack isn't the only thing they fixed. Most IOSs had this bug as well and those have now been replaced by newer versions that do not have the Trucha bug anymore. Due to this, we could no longer use any Homebrew that relied on the Trucha bug being there.
The answer to this problem at that time was simple: create some Apps (called Downgraders) that are capable of re-installing the older, trucha bugged IOSs. In other words, they allowed you to go back to the latest System menu with the bug still intact: 3.2.
This all went well for quite some time, and many people were happy with their 3.2 Wii's. In the mean time, an App named Starfall was released which could effectively Block disc updates so people would be able to keep the 3.2 firmware.
However, Nintendo wasn't done with us yet! After some time, they managed to bring out a new kind of Wii, the Lu64+ Wii as it is called now days. These Wii's have a small hardware change, as well as an update for Boot2. These changes made it so that when a Lu64+ is downgraded, it instantly Bricks the Wii. (To find out why, read About:Lu64+).
Ok, so we know that there are only few IOSs that can be safely downgraded on Lu64+ wii's. Can't we use one of those to do something?
Yes we can, and we even still use this method today!
The principle is quite simple: Downgrade one IOS that is safe to be downgraded (like IOS15) and use that IOS to patch the Trucha back in a more recent IOS (IOS36, since that's the most suitable one), which could then be used for other stuff. This is how the Trucha Bug restorer was made!
Using this new method, it was no longer needed to Downgrade your Wii to a lower firmware, as you could simply restore the Trucha bug on any firmware version!
Conclusion: We don't downgrade our Wii's anymore for 2 reasons:
It is not even needed!
It is dangerous on New Wii's.
So the next time you see a guide that uses Downgrading (I mean the full downgrading, not the IOS15 downgrading), please avoid it at all costs and try to warn other people so they would do the same!