The Aquastar Operations Manual
A column wheel is the wheel/gear which coordinates the start, stop and return to zero functions of the Chronograph hands.
In a standard quality chronograph, the column wheel is replaced by a system of cams. So, in oder to operate the chronograph, cams are needed to be layered in two pieces. One layer receives the input from the pusher, while the other transmits the motion to the clutch, where the connection between the chronograph mechanism and the movement is acheived . the cam also blocks the reset mechanism while the chronograph is in operation in order to prevent movements jaming.
A column wheel has exactly the same function, but is built in a way to provide more accurate, more precise operation, which can be noticed through the motion of the crisp and precise pusher action. Where the cam is made of two layers of metal, the column wheel is a single piece, and usually needs high precision machining to manufacture.
Beyond the precise feel, a column wheel provides more benefit. It is commonly know that cam-actuated chronographs experience slack when started, this is usually noticed when the chronograph seconds hand skips forward, but this is thought to be more an issue with the use of horizontal versus vertical clutches (where the chronograph’s mechanism meshes horizontally rather than vertically with the gear train), with the more common cam-actuated chronograph typically featuring a horizontal clutch and the column wheel variety a vertical.