Complex sentence: It was not until December 1942 that the demand valve was definitely improved in the way we know nowadays, when Frenchmen Jacques-Yves Cousteau (navy officer) and Émile Gagnan (engineer) met for the first time in Paris.
Split into simpler sentences: It was not until December 1942 that the demand valve was definitely improved in the way we know nowadays. This came about when Frenchmen Jacques-Yves Cousteau (navy officer) and Émile Gagnan (engineer) met for the first time in Paris.