A Sine Vise can be used with either manual or CNC machinery to securely position a workpiece surface at a precise angle to the cutting tool. Using a Sine Vise can make a lot of difficult work holding jobs easy, and is often the only way to drill holes at a precision angle.
Once you know how, Sine Vises are a snap to set up. All you need is a set of gauge blocks and a calculator.
This video from Tormach shows how to do it properly.