You're Correct!

An on plane golf swing means that the golf club has remained on plane during the back swing, down swing and through the impact zone. It can be best identified when the butt of the club faces the target during the down swing and will create a straight shot if the club head points straight at the target at impact.

Ben Hogan made famous the idea of an on plane golf swing using the simile of a glass pane from the golf ball to sit on your shoulders and you have to keep the club underneath it so not to get steep. His idea was that the left arm would sit across the shoulder angle at the top of the back swing so that the swing was flatter and not too steep.

There are 3 different types of plane; steep, flat and on plane.

A steep swing plane is when the left arm gets too close to the back of a players neck and causes the butt of the golf club to point left of the target on the down swing, therefore making the club cut across the ball from out to in which would encourage a pull or sliced shot.

A flat swing plane is when the left arm gets too low at the top of the swing, causing the butt of the club to point right of the target during the down swing to create and in to out swing path which would encourage a push or hooked shot.

A straight shot and an on plane golf swing is near impossible to repeat because the club face needs to come into the ball at 0.0 degrees relative to the target with a 0.0 degree face angle to the target. This is why all golfers from tour standard to beginner level, have a specific shape of shot. For example, former world number one, Martin Kaymer, hits a fade and current world number one, Rory McIlroy, hits a draw. All swing plane are relative to the players height, build, flexibility and swing preference. But having an on plane swing is a great target to work towards.

An on swing plane means the ball will travel straight with no curvature so due to this swing path the ball will not start right of target and shape back. It can only pull hook if the club face closes to the target at impact.

To fade the ball, the club face needs to approach the ball from out to in relative to the target line, however, with an on plane golf swing this would not be possible as the club approaches the ball straight to the target line.

Swinging the club on plane will not necessary generate more power as it only affects the direction of the shot, however, a good strike and trajectory will occur if you change your swing and manage to swing more on plane than you once did.