The difference between downside and semi-deviation is only that downside uses the average return as a target while semi uses the target rate specified.
Both calculations account for periods during the time series of data when the return is below this target and the variance thereof to calculate deviation. This differs from standard deviation which looks at all variance of returns both up and down.
The only circumstance where downside or semi deviation can be zero is when all periods in the time frame have returns greater than the target rate i.e. a continuous uptrend.
