article_snippet.sent: What wasn't known is what causes each discharge to be pulsed, like a machine gun. '
article_snippet.sent: Previous researchers suggested that the pulses were caused by muscle contractions or by a fluttering of the exit duct during the explosions.'
article_snippet.sent: A stock image of a machine gun is shown .
article_snippet.sent: A stock image of a machine gun turns out the pulses are generated in a passive way, not through an active process involving muscle contraction, as previously thought.