Mine's a round disk with a hole in it; was in the car when I got it, and haven't messed with it yet.
The idea is to keep the water in the radiator longer so it rejects more heat; obviously the other side of the equation is keeping the water in the motor too long and having more heat to reject.
With iron heads, my car ran quickest/fastest with 120-degree water, (warm oil); I honestly don't know what the current aluminum heads on it want, but I haven't experimented with that yet.