At one point just before ejaculation, the controller (read: the one holding the "cock") can choose to put a finger, thumb or round object posed just above, or to the side of, the urethra of the penis. Upon exiting the urethra tract, the semen contacts the finger, thumb or round object and because of the obstructed path from said object, the ejaculated semen sprays like a "dirty" hose. This can be a solo, partner, or group act.
"keep talking like that and you'll get the dirty hose"
by Wallace Krumpledorf March 03, 2012