This phrase originates from the feeling of pure unadulterated joy associated with the ball hitting the back of the net in football (soccer in the USA).
Should be used when fortune has fallen upon one's path
After retrieving the lost Inca Golden Headed statue from the hidden Jungle temple, whilst escaping deadly spiders, poisonous arrows, and out running huge stone boulders, and it must be said keeping hold of your hat, you raise the statue high in the air and exclaim to the masses of indigenous tribes;
"Back of the Net"
Used by a manipulative bastard, after scoring a chick he doesn't deserve - a real player
"he shots, he scores, back of the nets!" he said to a girl, whilst walking out the door, after a randy one night stand