I don't have much idea on this and never used it but still I believe that this works with the trend lines option available in some chart. (line, bar, combo and scatter).
Those trend lines are statistical computation (executed on the expression) and on those trend lines forecast or backcast can be applied.
To get forecast or backcast just give intervals for them in axes tab so for example you give 2 for backcast and 3 for forecast then that trend line will be drawn 2 steps back and 3 steps forward on the x -axis data points.
on this topic some other discussions are also available, please see them to get a more clear idea.
Search Forecast or backcast on the community to see more posts, as well as also search in the qlikview reference manual, there also some description is available.
