This question was already answered on poolcenter(dot)com here
and at TroubleFreePool(dot)com here
. A sequestering agent combines with metal ions to hold them into solution.
The purpose of ascorbic acid for removing metal stains, especially iron, is to dissolve the oxide of iron (which is the stain itself) via the acid and to reduce it to a more soluble form (ferrous instead of ferric) that is better held by a sequestering agent. So to use the ascorbic acid, one must lower the Free Chlorine (FC) level in the pool to below 1 ppm (or to zero) so it is best to add some PolyQuat 60 algaecide before letting the chlorine drop in order to prevent algae. The same principle applies to copper stains except that they do not need to get reduced (but the acid from ascorbic acid is still effective at removing the stain when combined with a sequestrant to keep it in solution).