You're Correct!

The intention in the down swing is to deliver the club on the desired path and attack angle. These elements accompanied by the desired club face position and strike location will then dictate the balls flight.

Like any shot in golf, we are trying to control the balls trajectory to hit the ball a certain distance.

Having hopefully arrived at the top of the back swing in a good position, the down swing will be made much easier to achieve. As you start down from the top of the back swing, the hips shift towards the target while the upper centre stays centred. As this happens, the hips rotate as the body then begins to unwind as the arms begin to lower. When the arms are parallel to the ground in the down swing, the shaft should be approximately on a 45 degree angle with the hands dissecting the trail arms bicep when viewed from down the target line.

This position means the club is on plane. Your head should have remained in the same position as it was at address while you keep your inclination to the ground. When the club reaches parallel to the ground, the club head would ideally be parallel to the target line, running through the hands and in line with the shoes toe line. This means the club is working on the correct plane and angle of approach, which assists you in delivering a club head moving on the ideal path, inside to square. From here, the body continues to rotate as the arms straighten into impact. If you can swing the club close to these positions, you are close to hitting great iron shots.

If the club head is attacking the ball from outside the ball to target line, you will likely be hitting a combination of pulls, pull hooks, fades and slices. With the club also attacking too much from outside the ball, the club head will be moving across the golf ball in a glancing fashion at impact meaning less energy from the clubs speed being transferred to the golf ball, resulting in inefficient distance.

When hitting an iron, you want the low point of the swings arc in front of the golf ball in order to achieve a ball before ground contact. By having too much weight favouring the trail leg in the down swing, theres a good chance the club will hit the ground before the ball unless compensations to shorten the club arc are made, such as bending and pulling the elbows apart. Keep your weight favouring your forward leg to assist in hitting solid iron shots.

The term under the plane is where the club head is travelling from underneath the angle of the clubs shaft you established in the address position when viewed from down the target line. If the club is travelling from under the plane, the golfer will hit pushes, draws or hooks. Achieving a club head working down the plane will assist in being able to hit straight iron shots.