When you are on a break away and no one is around you, when you make it within the 10 yard line release the Analog-stick. Your player will still run it in, but he will add a little attitude to his walk. When you release the Analog-stick determines what way he will come in. Be careful -- if someone is nearby they can tackle you and you will have a higher chance of fumbling.
Before scoring a touchdown, make sure there are no defenders close to you. When you get between the 10 yard line and the endzone, release the Analog-stick. Your player will celebrate into the endzone. Also make sure you are not too close to the sidelines are your player may step out of bounds. If there are defenders nearby your player can fumble the ball if he gets hit, especially if he has low ball handling skills. Enter practice mode, Offense Only, and try it.