There are times when it is convenient to draw paths outside the actual image canvas, a good example of this is when you need a nice smooth curve that goes off the edge of your image - those curve handles can get very tricky to manipulate when they get very short!
Longer handles on you bezier curves give you more freedom when altering your curved path, allowing much more precision. Simply switch to full screen viewing mode (hit F) and pull your path point off the canvas. Now you can get much more precise control over the handle on the bezier curve.
Just be sure to save your path, the simplest way to do this is drag your path in the Paths Palette down onto the ‘new path’ icon which looks just like the new layer icon at the bottom of the palette.
Your path or vector shape will automatically get cropped to your canvas size for output, but will always be there and editable in Photoshop if you enter one of the full screen viewing modes.