The Wii U console and gamepad communicate via 5 GHz 802.11n WiFi, but in order to deter other parties from simply hopping onto the access point, Nintendo slightly obfuscated this WiFi standard. Specifically the WPA authentication was modified by a byte swap in the PTK, rendering every existing WiFi stack incompatible with the Wii U.

Knowing this, the key is to use a platform that allows one to pre-break WPA in a similar fashion, such as is possible on e.g. Linux and BSD. Along with the use of the hilariously insecure WPS that is triggered when the gamepad’s sync button is pressed, this enables one to connect a modified Linux system to a Wii U console. After this the console starts sending h.264 (AVC) encoded video to the ‘gamepad’, and a binary packet can be sent back with the controller inputs.

Suffice it to say that this finding was immediately turned into a GitHub project called Vanilla Wii U, that enables a Steam Deck to be used as a gamepad, as well as any Linux – and presumably BSD – system with a compatible WiFi adapter.