chevalier,
The popular explanation is that, like the "steel apples" on European blades, the balls add a bit of momentum via centrifugal force by sliding forward during a swing.
More about these:
http://forums.swordforum.com/showthr...ight=afflicted
http://forums.swordforum.com/showthr...hlight=chevron
Apparently there is debate on whether or not these balls actually serve a practical or decorative purpose.
http://forums.swordforum.com/showthr...ight=afflicted
Hope this helps in some way.