The IEEE 1394 standard is better known by Apple's "Firewire" nomenclature. Firewire 400 allows for both a small 4-pin and a larger 6-pin connector.
The additional 2 pins on the 6-pin Firewire 400 connection are used to provide power to portable devices. Many self-powered devices, such as digital video cameras, use the smaller 4-pin connector.
This adapter provides a male, 6-pin, Firewire 400 connection on one side and a female, 4-pin connection on the other end. This allows you to use a 4-pin (iLink) M/M Firewire 400 cable to connect between your Firewire 400 equipped computer with 6-pin connector (such as older Macs and PCs with a firewire card) and your 4-pin equipped device (e.g., camera).
Note that if you do not already have your cables, you might just want to get one of our 4-pin to 6-pin M/M Firewire 400 cables.
Also, if you have a 6-pin M/M cable that you need to connect to a 4-pin device, you need PID 360 instead.