Blender 3D: Noob to Pro/Advanced Tutorials/Advanced Animation/Guided tour/Const/st< Blender 3D: Noob to Pro | Advanced Tutorials | Advanced Animation
Stretch To causes the affected object to scale the Y axis towards a target object. It also has volumetric features, so the affected object can squash down as the target moves closer, or thin out as the target moves farther away. Or you can choose not to make use of this volumetric squash-'n'-stretch feature, by pressing the NONE button. This constraint assumes that the Y axis will be the axis that does the stretching, and doesn't give you the option of using a different one because it would require too many buttons to do so.
This constraint affects object orientation in the same way that Track To does, except this constraint bases the orientation of its poles on the original orientation of the bone! See the page on Tracking for more info. Locked Track also works with this constraint. Options
Pressing the R button calculates the rest length as the distance from the centers of the constrained object and its target
Rest Length determines the size of the object in the rest position
Volume Variation controls the magnitude of the effect
The Vol: buttons control along what axes the volume is preserved (if at all)
The Plane buttons define which local orientation should be maintained while tracking the target