[YodasLuke]

the golfer starts the downstroke by uncocking the right elbow through the muscles of the right forearm the pivot responds with a sliding action...then a rotational action during the release phase.
DG

Above Bold, Italics, and Underline by YodasLuke.

Since we're all in agreement that the answer for every question is "the Magic of the Right Forearm", the following will most certainly be a mute point.

The posterior compartment of the arm is an anatomic compartment which contains muscles which are all supplied by the radial nerve. This compartment is also known as the "extensor compartment", extension being its main action.

The muscles of this compartment are the triceps brachii and anconeus muscle.

The triceps brachii is a large muscle containing three heads (lateral, medial, and middle). The anconeus is a small muscle stabilizes the elbow joint during movements. Some embryologists consider it as the fourth head of the triceps brachii.

Additionally, by starting the downstroke with the straightening of the Right Arm, you will succeed in becoming the shortest player in your group.

Happy G.O.L.F.ing