How does Thomas experience some of the gas and what happens when a German soldier sees Thomas?

As the gas comes in, Thomas falls and his mask comes off some, allowing a little of the gas in. Thomas breaks and runs from the gas. He ends up in a trench with his gas mask off, retching. He looks up to see a German soldier with a rifle aimed at his head. Thomas has no gun and is sure that today will be his death. Instead, the German tells him to run. Thomas doesn't know why this German decides to spare him, but he makes good use of the opportunity.