To summarize https://files.catbox.moe/5679d4.jpg If the downstream pressure is approximately half of the upstream pressure, we achieve choked flow at the throat of **any** duct. When this happens, the pressure downstream drops, just like sand passing through an hourglass. This is because the air gets choked by the supersonic shockwave that occurs just after the throat. There are two ways to create this pressure imbalance, we either increase the reservoir pressure stagnation pressure, or we lower the downstream back pressure. Or the two combined. This law comes into play with the new F1 cars which are essentially ground effect cars, with two huge venturi tunnels. The goal becomes two fold, create choked flow at the inlet of the tunnels, and create two strong vortices to lower the pressure behind the throat of the tunnel.