This board is a unique board recommended by a SparkFun customer. When playing with SD, you sometimes need to view what the SPI traffic is doing, whether for hacking or debugging. The SD Sniffer allows you to hook up an external device, such as a logic analyzer or microcontroller, to view what signals are being passed back and forth. Insert one side into a regular SD socket and a SD card into the SD holder, simple as that. Project description, more information and ordering can be found at https://www.sparkfun.com/products/11468