Some quick advice when it comes to DS flashcarts:
- NEVER buy a flashcart with a year in the name. These flashcarts are intentionally rigged to stop working in a couple of years to get you to buy another one.
- Unfortunately, counterfeit flashcarts are very common, and finding reputable sites is difficult. This one is actually reputable despite it looking ridiculously shady; that's just kinda how these go.
As for the subject of DS flashcarts and their relation to DS and 3DS homebrew, I'm going to break this down for you completely, because you may have read scattered information here and there that you couldn't make complete sense out of:
A flashcart is a rewritable device that imitates a normal cartridge, allowing you to run ROM images. In this case, you're talking about a DS flashcart, which lets you play DS ROM images, such as commercial DS games or
DS homebrew. It is true that most (though actually not all) DS flashcarts are blocked by the 3DS home menu, so you would normally need a DS or older firmware 3DS to run them.
This doesn't have much to do with the subject of
3DS homebrew, though. Over time a number of exploits have been found on the 3DS, though most have been patched. Some are only userland exploits, which means they're enough to run user code such as the Homebrew Launcher and various homebrew programs, but not enough to have full control over the system. Other exploits, however, are enough to take over the whole system and install what's called "custom firmware" or "CFW". This has the power to do things like create and restore full backups to unbrick systems, install homebrew as full titles on the Home Menu, set custom themes, add cheats to games, and so on.
One CFW exploit is very notable because it is
unpatchable, exploiting a hardware feature of the 3DS rather than part of the firmware. It also involves using a DS flashcart, which is where they become relevant when it comes to 3DS homebrew.
To test 3DS systems at the factory before shipping them off, Nintendo left in a particular feature in the 3DS boot ROM. If a special kind of DS cart is in the slot, the system lid is shut, and the X + Start + Select buttons are held down while powering on the system, it will boot into that cart immediately with full system privileges. This exploit is called "ntrboot", because a DS cart is used and the internal codename for the DS is "NTR" (whereas DSi is "TWL" and 3DS is "CTR").
ntrboot had two main hurdles to cross. The first is that the buttons must be held down while the lid is detected to be closed. This would normally be difficult, but it turns out that the 3DS detects this by using a magnet, and holding a decently strong magnet near the ABXY buttons is enough to trigger sleep mode. The other hurdle is that a special kind of DS cart is needed, which would make it very expensive if they had to be manufactured from scratch. However, it was discovered that
certain DS flashcarts could have their firmware rewritten to work as this kind of DS cart, and DS flashcarts are fairly cheap. With these two hurdles circumvented, it became a fairly easy and affordable exploit to pull off, if no others are available.
There is one other difficulty, though, which is that you need to be able to rewrite the flashcart's firmware in the first place. This can be done with another 3DS that already has CFW, or by running a special DS ROM on the flashcart itself that rewrites the firmware. This means that if you need to use ntrboot, have a flashcart that is blocked by the Home Menu, don't have another CFW 3DS, and don't have anything that can use the flashcart normally, like a DS, you're out of luck unless you can borrow someone else's DS for a few minutes.
Now, you may be interested in installing CFW, but there are some risks to be aware of. By doing so, you run a risk of being banned from online services, such as playing games online, downloading/uploading SmileBasic programs, and so on. CFW actually will let you unban yourself to some extent: each 3DS has a special file called the friend code seed, and by getting one from another 3DS, your system will no longer be banned, though it will result in a new friend code. The problem, though, is that both 3DS systems would now have the same friend code seed, so another ban would ban both systems. Although some people share friend code seeds online, meaning you don't need to buy another 3DS yourself, a lot of people may end up using the same seed, and it only takes one person being caught to ban all of the people using that seed.
If you are still willing to take that risk, you should follow
this guide and no others. Any other guide is liable to be outdated or unsafe. Also, note that for the current firmware (11.6.0) the only exploits are ntrboot, transferring hacked DSiWare from a CFW 3DS via System Transfer, or physically modifying the system, so chances are you will need a ntrboot-compatible flashcart.
Alternatively, you could just stick to DS homebrew, where there's no risks to worry about in terms of being banned.
