To quote from the press release (2014)
Due to its need for a frequency of 360,000 semi-oscillations per hour, the Montblanc TimeWalker Chronograph 100 cannot rely on the same mechanical principle as a conventional chronograph calibre, which vibrates at a much slower pace of 2.5 or 4 hertz. This necessity prompted the watchmakers in Villeret to devise a solution with two balances. A large, massy, screw balance oscillates at a frequency of 18,000 A/h (2.5 Hz) to ensure the precise rate of the going train, while a small balance paced at the extremely high frequency of 360,000 A/h (50 Hz) regulates the chronograph function. Rather than remaining continually in motion, the separate balance for the chronograph begins to vibrate only when the “start” function is activated by a flexible steel lamella (called fouet) mounted behind the arrowhead of the chronograph rocker. When the “stop” function is triggered, this slender steel plate arrests the tiny 50-Hz balance and holds it motionlessly in place until the next elapsed-time measurement begins.
The chronograph function is powered by its own barrel, which stores enough energy to measure intervals up to 45 minutes in duration. An elapsed-time measurement can be indefinitely extended by turning the crown anticlockwise while the chronograph is running, thus adding fresh energy to the chronograph’s barrel. The going train for the ordinary time display draws its energy from a second barrel that guarantees a 100-hour power reserve.