hi there jezbond,
I think you are refering to a small hinged item that clips shut with a hole down the centre when closed.
If this is the case then it is used to reduce electrical interference on the firewire cable.
the usual way to thread the wire is to place the wire once through the hole down the middle then wrap it around the outside and through the hole again so that when it is clipped shut you have the cable running through the hole, looped out and back through again.
If this explanation is leaving you befuddled i can probably come up with a picture or diagram or something.
Some people prefer to have one on each end of the cable about 3 inches from each end.
Fine if you have two cores but if only one is fitted i prefer to place it near the end with most inteference ie the computer end.