This venturi tube funnels gasses from the propellant rearwards and over the shooter's shoulder, similar to that of a recoilless rifle.
ðɪs vˈɛntʃɚɹi tˈuːb fˈʌnəlz ɡˈæsᵻz fɹʌmðə pɹəpˈɛlənt ɹˈɪɹwɚdz ænd ˌoʊvɚ ðə ʃˈuːɾɚz ʃˈoʊldɚ, sˈɪmɪlɚ tə ðæt əvɚ ɹᵻkˈɔɪlləs ɹˈaɪfəl.