Consider a gas enclosed in a container which is half vacuum. If the wall is replaced by a porous plug (or a small opening sufficient to allow the gas to pass), the gas will rapidly start to move towards vacuum. Hence, no time will be allowed for heat exchange. And hence, the gas will expand adiabatically. This expansion is called throttling process.
The above setup describes Joule-Thompson porous plug experiment.
However, for ideal gases, the temperature for ideal gases before and after throttling remains constant.
In case of real gases it may vary.
It can be determined using Joule-Thompson coefficient (μ) : It is the ratio of change in temperature per unit change in pressure for an isenthalpic process.