When a venturimeter is inserted into a potential flow, the pressure inside is influenced by Bernoulli's principle, which states that pressure decreases as velocity increases. The design of the venturi creates a pressure gradient, with higher pressure at the inlet due to ram pressure and lower pressure in the narrower section, facilitating flow. The discussion clarifies that venturis used for measuring aircraft speed do not create a vacuum but rather measure dynamic pressure differences. Additionally, it emphasizes that the flow into an open-ended tube occurs due to the pressure differential created by the surrounding fluid movement. Ultimately, the pressure dynamics around the venturi and pitot tube are crucial for understanding fluid flow behavior.