the point after mind over bladder fails and at which you have to piss so bad that to relieve pressure on your bladder you relax allowing pee to enter your weiner but you hold it by squeezing the end of it (with your fingers) until you can whip it out and take a proper piss.
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".
(on a road trip)
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