The heart shaped boxes uses a unique description for the shape of the heart. Look at this video to get an idea of how the 'tilt' parameter and the shape parameter work together to define the basic shape of the heart.

Under the hood, the heart is composed of a rotated ellipse, that is mirrored horizontally. The 'shape' parameter determines the lengt-to-width ratio of this ellipse, while the 'tilt' parameter determines the rotation.