Oh, btw, when should I move my wrist, even though my arm makes the movement?
I would first like to address the question on wrist movement. The poster C Bass is correct that it is impossible to describe. At least that is what my instructors have always told me.
I can share with you some facts and mental approaches that have worked for me.
fact: its not matter of when, because the wrist is loose, it flows as extension of the shoulder, which is where all bowing must come from. Power and agility of are a function of shoulder. So its really not an issue of when should you move your wrist, that's entirely not a good mental strategy.
What is a good mental approach is just to think of the wrist as being "unlocked" in fact this entire process is called unlocking the wrist and will take a decade to master(I am not a master of flawless bowing technique).
Fact: It is mandatory to practice bowing on each open string in front of a mirror, and seriously do not try and make any movement of the wrist on your own, when the wrist and mind have been unlocked it will happen.
So remember the goal is not to move the wrist at the correct time it is rather to unlock the wrist along with the mind.
Fact: It is ok to drop your bow. In fact it is a good thing, it is a sure sign your hand is gripping the bow with the proper distribution of force. The bow must be loose in the fingers and hand. This is necessary for the wrist to flow.
The reality is you never will drop the bow, if your thumb is bent and properly located in the curve of the frog(this is the proper place regardless of what older publications may say). Again its a mental thing, you have to feel as loose and as free as you would if you didn't really care if the bow fell. Or even if you were trying to hap hazardously drop the bow.
The last thing I would like to add to this is do not focus on tone when gunning for wrist technique, the light pressure and tension free grip will not produce good tone....AT FIRST, in this stage your goal is simply to unlock the wrist.
Do practice/warm up sessions of just bowing, then be musical and have fun. each time you focus on pure technique for short periods of time, it will slowly creep its way into your playing, when your focus is musicality and not bow/wrist tech.
Finally you must go see a symphony orchestra and sit in the front row with a good head on view of the cello section(bass bowing is hard to see from anywhere in a hall) and cello wrist movement is close enough to give you an idea of what it is you are trying to mimic.