Preserve total means that a forecast’s total spend will always be preserved. With this option selected, when the phase spending values are manually edited, the other phasing values will change accordingly so that the sum of all phasing values still equals the total spend.
If all unlocked phase spend values are edited, and the sum no longer equals the total spend, when saved, the phasing values will change to equal the total spend, but will be weighted according to how the user edited the phase spend values.