My 20 cent's worth!
Make sure your bow hold is relaxed. Test this by using your other hand to gently tug the bow in each direction and observing your fingers, thumb and wrist take up the slack. Then place the bow on the string and, without moving sideways and making a sound, use the hair gripping the string to test again how relaxed your bow hold is. If you kept pulling in either direction the string would release itself from under the hair and sound will start. All this just sensitizes your bow hold to the flexibility of the bow, hair and strings. Each string is different because of thickness and weight.
Trying for a smooth bow change (ie Legato) is IMO trying not to disturb the way the string is vibrating as you "turn the corner" (change direction) at each end of the bow movement. Picture that the last things to change direction are your fingers holding the bow and that everything else (hand,wrist lower arm and upper arm) ideally has already just turned the corner first.
The advice above about using a mirror is good, so is being calm and steady (no sudden jerks or accelerations or variations in weight). Look down at the width of string movement and study how each string moves according to how you change the three basic variables, namely bow speed weight and distance from the bridge.
Practice making flowing artistic arm movements without the bow in your hand . These can be much more florid than when bowing on the string, so that you see and feel the hand and fingers follow through in a controlled "flap" after the rest of your arm changes direction. These can be done "in the air" or with your finger tips resting on a table top.
What all the above adds up to is the bow is the last thing to change direction. The slight finger and wrist movement is caused by the momentum of the bow as it follows through. If the wrist and finger movements are not dictated by the string and bow they run the risk of being too florid and will cause a disturbance in the sound. An artistic arm movement will produce an artistic sound if you get the correct sequence of events happening
My concept of a perfect legato bow change is that it turns the bow hair and sound from a series of straight lines into a seamless circle of movement and sound.
So, look down, observe and listen, and work backwards from the string to find solutions to bowing problems? And, an artistic relaxed-but-not-floppy expressive hand movement will produce an artistic sound
Happy hunting!
DP
