From the game NBA Street Volume 2 for the Nintendo Gamecube
First, have someone perform a Short Range or Long Range shot (it's much easier to execute using short range). Hold down the "shoot" button as long as possible so that you shoot the ball right before landing on the ground. Right after letting go of the ball, run to the net and perform a Tip Slam (L + R + Y). If done correctly, the ball should go into the net, scoring you points, then you should grab the ball and then perform a Tip Slam, scoring you another point.
Submitted by: Zanto