Start at the point of the bow with an exaggerated bow-change action and maintain this as you work your way towards the heel.
As you change from the upper half to the lower half, the active part of the arm changes from the forearm to the upper arm.
When playing at medium or fast tempos the hand and finger movement for the bow change is passive. That is, in the upper half of the bow the forearm leads and the hand and fingers follow. In the lower half the upper arm (think elbow) leads and the hand and fingers follow.
The bow-change action is important for many strokes (fast détaché, forearm sautillé) and should be able to be accessed in any part of the bow. It is particularly important at the heel, which for beginners is a difficult place in the bow because:
It often takes longer to develop a freely swinging upper-arm stroke in the lower half than to develop a freely swinging forearm stroke in the upper half.
The weight of the bow at the heel challenges the bow hold.