Transition is the stage when the club changes direction at the end of backswing (BS) to prepare for the downswing.
Speed of Transition movement: It is similar to the moment when you drive a car to a roundabout, you need to slow down before speeding up. At the stage of backswing, your clubhead is having inertia of moving up, so it cannot abruptly stop to go down.
How to transit from Backswing to Downswing?
When your hip rotates to the limit of the BS, the lower body including feet, thighs and hip should prepare to help the body rotate back quickly. It happens while the block of chest, arms and club is still moving up (short distance only) as the inertia of BS.
Step 1: End of backswing and prepare for Transition
Step 2: Transition
Step 3: End of Transition