How to achieve smooth bow changes at the frog

On the cello, one might be told to lead the up bow with the wrist and then pull the down bow with the arm on the bow change. The arm is leading in both directions with the hand and fingers following.

But what about that momentary break in the sound, the slight accent one hears because of a quick flick of the hand? In trying to get a smooth, continuous sound, one needs to have the pressure or contact on the string remain constant, with the bow speed the same on the change. This is not so easy to control.