How Quant Arena calculates it
We divide the average daily return by the standard deviation of the days with a negative return only, then annualize with √252. Days with a gain do not increase the denominator, so a strategy with large up days is not penalised for them.
When it differs from Sharpe
Trend-following strategies often have a Sortino well above their Sharpe because their volatility comes from a few large winning moves. If Sortino is close to or below Sharpe, the losses are as large as the gains, which is typical of mean-reversion or option-selling style returns.