Morandir, I think all Plarkin is trying to say is that both sources of air intake on the tube converge as soon as you close your lips around the mouthpiece and apply negative pressure. Since all openings to allow air to enter the tube lead to the single air pathway being inhaled, they are directly correlated in an additive manner, and therefore, a change in pressure can result from alteration of either air input location, which can affect both draw and switch activation. Scientifically he is right, the problem is observation, since we cannot maintain a constant negative pressure at the mouthpiece to measure the effect - humans adjust their draw based upon feedback from the system, and it is difficult to rectify theory with practice. Not trying to dispute your many findings, just trying to provide some outside rational scientific support for what he is trying to say..... I think 
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