Thank you for asking. FLD "Forward Line of Demarcation" is a term from the Hurst people that denotes the price at which the MA of the designated length will change direction, ie slope will change from negative to positive or vice versa.
Why is that relevant? If prices are following a sine wave pattern with wavelength L, a moving average of length L remains unchanged at value zero. A moving avg of length L/2 will lag the movement, and change direction at the zero point, or halfway from bottom to top. That allows a projection of where the top is by knowing the low and the halfway point. When current price exceeds the price L/2 in the past, then the slope becomes positive. If the price from L/2 in the past is plotted at current time, the resulting line is known as the FLD(L/2).
Stockcharts will plot prices or ma's displaced either forward or backward in time. Parameter of (1,20) means data point is moved forward 20 time periods. (1,-20) moves it backwards. (9,20) takes the ma9 current value and moves it forward 20 periods.
Let me know if you have further questions.
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