think of a firefighter hooking up a fire hose to the fire hydrant. before they put out the fire, they have to open the valve and pressurize the hose, hence "charging the hose".
A. Pull over, I have to piss so bad!
B. How much time do I have?
A. None at all, I have to charge the hose!
B. Shit, you better grab that empty water bottle then, you are in deep trouble