That straight-line planar relationship must be maintained during the early downswing and mid-downswing - while the clubshaft shallows-out during its passage to, and through, the "slot box".
A right arm swinger starts the downswing with an active movement of the power package, while allowing the pivot action to be secondarily reactive to the upper body/arm movements.
Note that his pelvis is still centralised between his feet - there has been no left lateral pelvic shift movement.His left elbow is bent, and his left wrist is cupped/scooped due to a bent left wrist, and that allows the baseball player to have his clubshaft more vertically aligned relative to the left arm.Also, note three biomechanical differences in image 3 of the downswing compared to image 3 of the backswing - i) note that the right upper arm is being actively adducted towards the right side of the torso in the downswing, while it is slightly separated.Standard slot swing - from reference number.Consider how Martin Hall keeps his clubshaft on-plane during both the backswing and downswing in this video presentation of the Smartstick device.In his book 1, bingo display board Jim McLean used the analogy of a baseball player to describe his idea of a club "fall-back" phenomenon.Sergio Garcia shallowing-out his clubshaft in the early downswing - capture images from his swing video The sequence of five photos shows Sergio Garcia shallowing-out his clubshaft in the early downswing.Now consider the movements of the hands/clubshaft with respect to these inclined planes.The angle of the right forearm relative to the ground is greatly affected by the angle of the straight left arm relative to the ground.It is the "correct" movement of the intact power package that causes the club to "correctly" drop into the "slot".Consider Sergio Garcia's hand/clubshaft movements in the early downswing.Image 4 shows Sergio at the end of the early downswing - when his left arm is parallel to the ground.The hand plane is angled through the hands at address.For most people, that's swinging a baseball bat.Image 5 shows Jamie Sadlowski at impact.
However, it is also perfectly acceptable to use the one-piece takeaway, because it will also allow a golfer to get to the same "correct" end-backswing position (as demonstrated in image 1).
This swing type is the opposite of the standard slot swing in the sense that the club swings under the address plane in the backswing and then loops forward to come down a slightly steeper plane in the downswing.
The pane of glass is then angled-up relative to the ground, and this gives rise to the concept of an inclined plane.