If you have a long torso, then tucking in your shirt works. To create balance, the goal is to visually shorten your upper half. Also, those who are long waisted usually have shorter legs. When tucking in your shirt, you give the appearance of longer legs. Yay! If you are short waisted (Do you fit into Petite clothes?), tucking in your shirt will make your waist even less visible. To avoid the ‘boobs then belt’ look, untuck that shirt (or wear tunic tops)!
In this picture, notice how the tucked in version gives the outfit a streamline look.