"'You see, it looks like an ordinary fan, but actually there are very thin plates of galvanized steel hidden under the silk. And when you bring them together...' He folded the fan, then brought it smashing down onto the desk. The wood shattered. '...it becomes a useful weapon. I call it...'
' ...the fan club?' Alex suggested." - Smithers to Alex Rider (Snakehead, page 162)
Thin plates of galvanized steel are concealed under the silk of a Chinese fan decorated with a graphic of two interweaving dragons. When the fan is folded the combined layers allow it to become a strong weapon.
The only known use was by Smithers himself, demonstrating the fan club to Alex at the MI6 temple base in the novel Snakehead.