You are basically pouring a solute (Ex: CaCl2) into a column and there is a cation in your column. Your goal is to precipitate the cation initially in the column by making it come out of the column and replace it with the cation you insert into the column (in this case Ca2+). Therefore, you need a cation that is less soluble with Cl- than Ca2+ is with Cl- because less soluble -> more likely to precipitate. For our case, we can use Na+. This will happen because once you pour the CaCl2 into the column and it dissociates, the Na+ cations and the Cl- anions will form NaCl precipitate and come out of the column and the Ca2+ will remain in the column. The net result is that you replaced the column with a different cation but this experiment shows you that less soluble -> quicker to precipitate.