The Flat Pass rulle allows a player to Pass the ball to team mate who is horizontally inline with him on the field.
It's a risky Pass as it requires a Dice Roll that is lower than his Ball Skill. So why take the risk?
Using the example above, you can see that the Fly-half has 2 players he could potentially Pass the Ball to player 8 or player 3. Passing to player 8 would be an easier Pass, but taking a chance and Passing the Ball to the number 3 would give that little extra distance to make it to the Tryline.
Risk vs reward, that's the beauty of the Flat Pass.