A rarely observed fact that can instantly change your bow arm!

I was working with an exceptional violinist the other day and discovered that she didn't know this important but little known fact of the bow. This is how the conversation went. I said to her...

What is the line or direction of the pull on the down bow?

Quickly we deduced that it is to the right and horizontal.

Then I asked her what is the direction of pressure?

After considering it for a moment it was discovered to be vertical or downward. Then we proceeded to the up bow.

What is the line of the pull on the up bow?

Almost immediately we realized it was horizontally to the left. But then came the punch line...

Now what is the direction of the pressure?

She was holding the bow at the tip and pondering the question. "It can't be downward vertical can it?" "Not really" I said. "Try drawing the up bow and feel what direction it might be." She tried it and it dawned on her. "It's horizontal!" "Yes," I said,

"the line of the pull and the direction of the pressure on the down bow are NOT the same but on the up bow THEY ARE THE SAME."

Does your bow arm feel any different?

I asked. "Yes, it feels more comfortable." she replied. "That's because it's natural" I said.