GD
Here goes... The yaw to the left, as you know comes from the increase slipstream corkscrewing over the fuselage and striking the left side of the vertical stabilizer. This effect is more pronounced at low speed/high power setting.
If you get the chance to look straight down on a light SEP, you'll get the answer. The vertical stabilizer is actually positioned at a slight angle off the centreline. Thus in cruise conditions, the yaw affect of the prop slipstream (trying to yaw you left)will be naturally countered by the built in 'twist' in the vertical stabilizer(trying to yaw you right). Should you suddenly reduce power, you have no slipstream but the vertical stabilizer (with rudder neutral) will yaw you to the right.
This right yaw is not as pronounced as the yaw to the left, and in the good ol'days, I'd apply slight right rudder to make my point. As a prudent pilot, you shouldn't be making these rapid power changes anyway, hence some pilots not noticing it, except during their PPL training days.
I never noticed it on the C208's, but then I wasn't looking for it and perhaps was just cancelling it without giving it too much thought. Obviuosly couldn't have been an issue.
PP