The SD-9 is an external battery pack for the Nikon SB-900/910. It has a total capacity to hold 8 AA batteries, but can also be used with just 4 batteries. It can be used with any type AA battery, but provides the fastest recycling time when used with 4 quality AA Ni-MH rechargeable batteries.
It doubles the number of flashes possible and shortens recycle time to as short as 1.o seconds when used with NiMH batteries (other types of batteries work, but a bit more slowly).
Note: this is a powerful tool. With it you can run amuck and ruin other flashes if you plug it into them. If you shoot as long and fast as you can, you could possibly burn out an SB-910. Use it carefully.