PSP development kits have an extra set of inputs and outputs: 8 on/off switches, and 8 LEDs.
These are called the GPI and GPO, respectively, and can be read/written using the PSP kernel functions sceKernelGetGPI/sceKernelSetGPO.
On consumer hardware, sceKernelGetGPI always returns 0 and sceKernelSetGPO does nothing.
From 1.18, PPSSPP can return custom settings for the GPI switches, and display the GPO outputs. This functionality can be reached in developer settings and in the in-game DevMenu, if enabled. As it turns out, some games actually do things with these, as can be seen in this YouTube video by PtoPOnline.