Building on that premise, the keeper covers two yards. If the keeper steps off the goal line on a line from the middle of the goal towards the shooter, that area outside the two yards the keeper takes up by standing gets smaller. When the shooter is at the middle of the goal, the most goal area is open. As the ball goes to the sides, the scoring area gets smaller if the keeper maintains the distance he is off the goal line. Angles are geometry and the principle builds on the keeper always being on a line from the middle of the goal to where the shot is coming from.
Shots can happen from the front stick and the reverse stick.
A keeper can't just line themselves up from the middle of the goal to the player taking the shot, it's got to be on a line from where the shot is coming from. When first introducing the concept of angles to young keepers, make the concept concrete. You can do it with ropes. Put a ball at a spot in the circle, get a long rope, tie it to the goal posts and pull the middle of the rope to the ball. When the keeper is centered between the rope, he's on goal.