From ZBrush Info

Real-time shadows provide yet another level of visual feedback as you sculpt. They don't require any rendering at all, and can be used anytime you are working on a mesh.

Different parameters allow shadows to be longer or shorter, deeper or lighter, or to otherwise vary to reflect the sculpting lighting you prefer, or the lighting conditions your model will ultimately be used with. Use them with materials, MatCap, textures, or polypainting to reduce the need to render as you work.

Real-Time Shadow Overview

To turn real-time shadows on make sure that the Render:Shadows switch is on. Settings for real-time shadows can be adjusted in the Render:Preview Shadows submenu:

Of course, the Render:Shadows switch still has the same effect in other rendering modes as it did in previous versions of ZBrush (i.e. it will cause full shadow rendering when a Best render is done.) The preview shadows setting allow you to set up real-time shadows in a way that will best reflect the final rendering environment.

Shadow Controls

The shadow controls are not intended to reproduce all of the settings that can be used to affect shadow rendering in a rendering engine such as Mental Ray. Their objective is to allow for the adjustment of shadows on different surface types, ambient environments, and so on, in a way that allows the sculptor to understand the visual effects of their sculpting as fully and as easily as possible. You should experiment with different combinations of the shadow controls to find what works best for you.

ObjShadow controls the intensity of the model's real-time shadows.

BackShadow acts as a drop shadow cast by the current model and projected onto the canvas. The slider value controls the intensity of the shadow.

Length increases the scan range that ZBrush uses to create the shadow. A larger Length will lengthen and soften the shadow but will also increase computation time.

Slope equals the azimuth of the light used to project the shadow. All shadows fall at a 45 degree angle across the canvas. This cannot be altered. Slope controls the angle in Y at which the light casts the shadows. Zero means that the light is directly overhead of the model. A larger number means that the light is more in front of the model.

Depth deepens and enlarges the shadows. The effect of Depth depends on the setting for Slope; at small settings of Slope, changing Depth will have little effect.

How Real-Time Shadows Work

Preview (real-time) shadows do not take into account the current light setup, because to do so would slow the rendering process and make real-time use impossible. Instead, the options for preview shadows can be thought of as affecting a virtual shadow light. Changing the settings for the preview shadow options will affect the appearance of preview shadows in different ways, allowing you to optimize your view for the the type of sculpting you're doing.