I believe Calico is correct also....the reason I said it was to keep the reel from spinning during a fight with a large fish is that is what happened to me on that charter. I hooked a citation sized (41 lbs) Wahoo and that very reel kept trying to spin on the reel seat when I would try to crank on the fish. The mate tightened the brace and that solved the spinning problem. Looking back, it was more than likely the result of a faulty reel seat and tightening the brace was a quick fix while fighting the fish.
FYI...here's a link to the picture of the Wahoo.
http://www.triumphowners.net/photos/...500/ppuser/531