The Super Gameboy adapter has a full gameboy inside and a custom IC that facilitates communication between the SNES and GB.
It converts the LCD signals from the GB into 2bpp tiles that the SNES can then DMA into its memory pretty quickly.
It also has two way asnyc communication using the joypad registers in the GB. This is used to (of course) transfer joypad input from the SNES controller to the GB, but also used to set up a command interface, allowing the GB cart to request special things, like palette changes or custom borders. Some of the commands require a little more data than you can cram through the limited interface, and these commands use the GB VRAM and LCD signal to quickly transfer a lot of data (up to ~5k each frame).
A lot of games made use of these features, but the command interface can do so much more. Interesting in particular are SOU_TRN, DATA_TRN and JUMP. These allow you to upload code into the SNES ARAM, SNES WRAM and jump the SNES PC away from the BIOS running on the SGB and start running your own code.
While running your own code you have full access to the SNES hardware, including the BIOS rom, which you can extract some assets from if you're low on memory
. The SGB communication registers and protocol is full documented, and you can maintain communication while running your code and fetch more data from the GB side if you want.
The GB CPU and sound continue to run alongside the SNES counterparts, so you could have 12 channel total polyphony if you want.
Only one game (Space Invaders) made full use of these commands, so this is proper obscure! Make some demos on it!