In the SDS-PAGE technique, the low concentration stacking gel is placed on top of the separating or resolving gel, which is more highly concentrated. The main purpose of the stacking gel is to line up all protein samples loaded on the gel, making it easier for them to enter the resolving gel at the same time. 

By ensuring that all proteins enter the separating gel at the same time, stacking gel allows for better separation based on molecular weight, resulting in enhanced resolution of electrophoresis. Without the stacking gel, proteins would appear as indistinct smears instead of distinct bands.