Released worldwide in 2006, the Nintendo DS Lite was a slimmer and brighter redesign of the Nintendo DS, a handheld console most notably remembered for making use of two screens and touch controls. However, one feature the Lite version could perform, which was to send video to a TV or other external display, was unused and hidden. At least until a team of hackers at Lost Nintendo History discovered it and brought this unused feature to life.
The project, known as the "Nintendo DS-TV-OUT Restoration Project," was formed in late 2020 when they discovered a leftover feature in the system on a chip (SoC) that could allow cheap hardware video output. However, the firmware in the Nintendo DS Lite automatically disables this feature early on whenever it boots up. The team managed to bypass this firmware through the use of custom firmware that's relatively easy to install and is "required only once." From there, they only needed "a few extra hardware components" to make the video signal from the DS Lite's hardware usable.
Aside from obvious uses for this feature, this impressive bit of reverse engineering is useful for troubleshooting problems or for using a Nintendo DS Lite when one screen isn't working by sending the signal from the non-functional screen. It's worth noting that this feature is only on the Lite version of the DS, not the original version or the DSi version, so it's likely that this was a planned feature that was dummied out later on. This sort of screen sharing feature wouldn't be officially used until the Nintendo Switch was released in 2017.
Lost Nintendo History have the installation instructions up on their website in case you have a spare Nintendo DS lite lying around and want to give this a try. Be warned though if something goes wrong Nintendo likely won't be there to help you out. The team says they're also working on creating a simpler process for more people to try it out.