dear ac.smith , thanks
I have learned quite a lot from your answer, however the stops issue is not clear to me. how do you decide how many stops you are below the correct one? please explain.
And quote you:
.........doesn't steady the preview image, but is theoretically more efficient because it minimizes the amount of corrective movement required by waiting until the instant the picture is taken - is perfectly effective when you're nearer to the correct shutter speed (and once you're within 1 or 2 stops it is actually more reliable)............
I certainly found it made handheld shots at 3, 4 or even 5 shutter speeds slower than normal perfectly possible. Impressive stuff (though the reliability is much better if you're within 3 stops),..................