Instructions
Move the ball up to as close to the end-zone as possible. Diving for the goal is only possible between 2 yards and inches from the goal line.
Snap the ball and hold the Right Analog Stick up towards the endzone. This will initiate the dive. Don't hold down the Right Trigger for "Aggressive Mode" as this will attempt to barge through defenders. Don't press the Left Analog Stick as this might interrupt the dive. There is another method with alternate controls as well.
Snap the ball and hold down the "X" button on Xbox 360 or the "Square" button on the PS3, again without using "Aggressive Mode" or touching the Left Analog Stick.