Originally posted by SteveB Wouldn't any rotation be around the axis of wherever the other hand is holding the non-shutter end of the camera?

Steve, it is a little more complicated than that. You see, camera shake is a misleading name. We commonly interpret shake to mean up and down motion or left to right motion of the camera, that is translation of the X-Y axes (the Z axis we don't count because that is in the direction of the subject and auto focus takes care of that)

But the shake reduction system doesn't measure this at all. What it measures is the forward up/down rotation around the X axis (pitch), the left/right rotation (yaw) around the Y-axis and the rotation around the lens Z-axis (roll). So all three measurements are rotations around the X, Y and Z axes, which we call pitch, yaw and roll respectively. Until the K7 only pitch and yaw were measured which made the camera vulnerable to roll induced by shutter depression.

So why do they measure rotation around the axes and not translation of the axes? In part it has to do with the way we point a camera. When we point a camera to keep the subject centered any short term deviations from the aiming point tend to be rotational in the form of pitch, yaw or roll.

In part it has to do with technology. We can readily make linear measurements of angular velocity using piezoelectric gyroscopic sensors. It is not so easy to make linear measurements of displacement at low frequencies where the axes are translated.

Sorry about the long answer. What I am trying to say is that all three measurements are about rotation and which one we call roll is a matter of definition. Pentax have defined roll as rotation around the lens axis.