Reverse S-Curve

Two main reasons for the Reverse-S Curve (1) Preserve the cusp and (2) Facilitates formation of 90 degree exit angle cavosurface margin, creating no undermined enamel.

The buccal and lingual extensions of the proximal box are not too wide and not to narrow. The extensions are just enough to have a clearance on the adjacent tooth.

While making sure that the buccal and lingual walls follow parallel to the direction of the enamel rods

There are two terms that you need to know about how this curve exits towards the proximal surface of the tooth: (1) Exit point, (2) Exit angle.

EXIT POINT is the exact location at which the angles are exiting. It is a reference point in which you want to check if you have formed a perfect 90 degree exit angle in this area.

The EXIT ANGLE is the direction at which the exit point it is exiting from. To check the angle, you just focus on the exit point. There will be angles that you need to look at. One on the side of the tooth and the side on the restorative material. When looking at these, you have to achieve a 90 degree exit angle. Simply saying when you have this feature, you have created an equal amount of tooth and restorative material on this area.

Lot of mistakes happen when dealing with reverse-s curves, a whole section is dedicated to common mistakes and how to do a better 90 degree exit angle.

Link to Common Errors and Prevention