Is an airsoft bb propelled by air or by the plunger impacting the bb?

the plunger acts as a venturi pressure block, which blocks the way after gas propelled the bb. so when its blocked the remaining released gas pushes the piston back together with the slide and activates a mechanism to close the magazine release valve, this time opening the plunger again and prepares it for the next fire. so in my opinion the piston actually is for the blowback action and not for bb propellant. the first answer by dip dip is for AEG guns, my answer is for GBB pistols. I can email you some diagrams explaining this.