Hi Kevinwd1,
Downwind controlling speed is easy and fun. You are probably used to riding upwind and edging hard etc. This is a good start but is very different to riding fast off the wind. We had a young lady who did last years Kitegirl Clinic couldn't ride downwind and did the walk of shame the opposite way to everyone else!
You need to ride nice and comfortably fast in your normal stance and simply allow your body to stand a little more upright and not edge as much on your heel rail. By putting a little more pressure on the toes, it allows your board to turn more downwind, but as you go the resistance to go upwind drops off, so you need to be a little more upright and start to move your weight very slightly back over your back foot.
To control speed, carve towards the kite or to the opposite side of the window to the kite, this instantly moves the kite high up into neutral without having to bring it to 12 o'clock by changiong your position in the apparent wind window. You will wash off huge amounts of speed and regain control all whilst throwing a nice spray to your heel side by digging in your toeside rail.
Sounds confusing, so imagine riding to the left with your kite at 45 degrees. Apply some toe pressure and stand up more over the board over your back foot. To control a runaway speed run, simply carve a toeside carve to the right without moving the kite.
Give it a go, you'll nail it easy, just practice and look in the direction you want to go.
Good winds,