They somewhat explain in the first game that there's a ring with a string attached to the bracer that with certain movements triggers the device. Back in Altier's day, with much more "primitive" designs, the ring finger beyond the first digit was removed to make room for the blade to deploy between the index and pinkie fingers; however, the "mutilated" ring finger still manipulated the pull-string mechanism for the hidden blade to activate.I took an interest in the mechanical design and, while flawed, is still an interesting design. Hopefully my explanation helps.