So, the second step in my lesson on chords, after learning to play them in the standard broken fashion, was to tackle the first bowing variation. This was simply playing each half note chord as four slurred eighth notes in the string pattern G-D-A-D, whatever the stopped notes on each string were.
The key to playing this pattern fast, and I assume all the others, is to learn to anticipate the changing bow levels across the strings with the upper arm leading the change. None of these patterns are played at the wrist, a common beginner mistake. The most entertaining part of the lesson was when T- had me pursuing function rather than notes, and I was flapping my bow arm like a goose trying to take off from the water. I had a good laugh, but that was definitely helpful.
So what's the trick? For me, it was dropping the arm downward from the shoulder before I moved the bow from the A to the D string. You can practice that slowly with the metronome to get the feel of it. On a down bow, put the beat on quarter notes (about mm=60), play open strings, and think: G and D and A drop D and, so the upper arm drops on the "and" of 3. Then, when changing the bow on the G string, be sure to drop to G before changing the bow direction.
This was the practice that made the most difference for me in Bach, going back the the 1st Prelude and taking a quick look at the 3d, inspired by PFS. Then today I read through the Martinu flute trio and found that, instead of panicking when I came to the broken chords, I found myself thinking "whew, bariolage, I can breath for a minute before I have to resume counting." Very worthwhile.
Shortly after my lesson I came across this video while perusing the blogs of random NaBloPoMo participants. The blogger wanted to discuss dreams, but I was excited to discover that this is a marvelous demonstration of that early shoulder drop in broken chords.