Complex sentence: A fallback will reset to zero and then continue to run when the reset button is pushed while the stopwatch is running, whereas most mechanical chronographs will reset to 0 only when the startwatch is stopped.
Split into simpler sentences: A fallback will reset to zero and then continue to run when the reset button is pushed while the stopwatch is running. In contrast, most mechanical chronographs will reset to 0 only when the startwatch is stopped.