Swing Your Bow Arm

The ability to play with the weight of the arm released into the string is wonderfully freeing. When playing in this way one feels ease and power. This is easiest to feel closer to the frog, but the feeling can be transferred throughout the bow. The following idea helps quite a bit:

Think of your arm as a swing. Swings generally have two ropes or chains that are attached to a bar or tree limb. Feel that your arm hangs from both your shoulder and the place where the bow meets the string. The elbow is the seat of this swing. Swings do not move in a straight line, but in an arc. Feel the same for your elbow, that it swings in an arc. As with a swing, the push/pull is initiated at the seat, not higher up the chain, and it is the elbow that should feel as though it pushes and pulls the bow as it moves (not the hand, for example). Especially important here is the released, heavy feel of hanging. A similar idea that I sometimes use is that you are balancing the weight of your arm in the string, and I like this one too, but here, hanging from the two places of the shoulder and string is the feeling we are going for. 
So, once you are hanging there, just push and pull from the elbow and feel your elbow swing in and out. If your elbow isn't swinging in an arc, you are probably not hanging from the shoulder. If you don't hear full, deep tone, you probably aren't hanging from the string. Your elbow probably will not actually be below the level of the string, and this is fine/good. If it drops too low, the weight will not hang from the string, but feel too far to your left. It is a great feeling when you find it, and a great sound as well.