[Schrödinger]

I was going through the Joule-Thomson experiment (Levine, Physical chemistry) and found that the pressure in the left chamber P₁ drops to P₂ in the right chamber, and this pressure drop is completely due to the porous plug.

Levine : "...Essentially all the pressure drop from P₁ to P₂ occurs in the porous plug".

My question is, what role does the plug play to reduce the pressure? I mean, isn't it just another membrane which allows gases to pass through? What is special about the plug that causes the pressure drop?
I'm really sorry if the question is dumb There might be a totally simple explanation for this, but I'm just not able to see it