Potential energy increasing in j-t effect

[GAGS]

Hi all, my question is very basic and i am unable to digest it. Actually while explainiing the mechanism of joule thomson effect we say that when gas is forced to expand through porus plug, potential energy of gaseous molecules increases. From basic definition of potential energy we know that where there is force,which tends to bring the system in original state(ex: spring system),there is hidden energy called potential energy.
But here when gas undergoes forced expansion force between molecules is vanderwal or weak forces,then how we can say potential energy increases and kinetic energy decreases.
Thanks