This is a passive skill and is gained automatically by the Navy Privateer class via the Uncharted Waters expansion pack.
When you disable a vessel in combat you will notice there is a button that allows you to Impound the vessel, doing so will give you an extra amount of money depending on the type of vessel and it will then cause the vessel to disappear so you can fight another one.
The Impounding skill does not go up, and will remain at the value it was when you started.