It's a time series estimation method developed as an evolution of Exponential Smoothing (ES) and based on two different smoothing constants, and then on two fundamental equations:

The first function determines an estimate of the level L_t-1 as a weighted average of the observed series last instant of time available and the expected value at time t-2.

The second function determines the trend T_t-1 as a weighted average of two estimates: the last date update for the trend and that obtained by the difference of the last two estimated levels.

The smoothing constants α and β can be determined by minimizing the error measure of prediction.

Editor: Giuliano DI TOMMASO