Pass through fire is firing that occurs during the defensive fire phase when an enemy unit moves through the arc of fire of one of your units. The rules I use state the you should make a 'mental note' of where you would like to fire at that unit. I'm not big on 'mental notes' so I use props to remind me.
A Confederate column begins its movement outside of the arc of fire of the Federal Battery.
During its movement the column passes through the artillery's arc of fire and end its movement once again outside of the arc.
Rather than trusting my memory and relying on 'mental notes', I place a marker at the spot I will be firing on the column during the defensive fire phase.
This works well for infantry firing defensive fire at a retreating enemy when you want your troops to fire before the enemy gets beyond short range.