On the right-hand side of the case, there's a flip-up lock for the chronograph. Right before you enter the water, you can start the chronograph and then flip the lock back down. The chronograph is then running, tracking the total link dive time with the chronograph hand and 60-minute counter. That's all powered by an automatic version of the AgenGraphe shown below. If you like link movements link that look incredibly complex, this is about as complex as you'll find for a "simple" (non-split) chronograph. link