which could be considered an "expansion" of the output, but really, you are slaving the output device to the master of the SPI bus.
Oh. I see. I've read somewhere that it's also used to burn the bootloader to the ATmega Chip. Is this true?
does this mean i can use it to cascade two shields? i'm having problems stacking two shields together (GSM Shield and LED matrix shield) because of pin connections.