Whether a reflection will diffract or not in a given situation is determined by several conditions that can be expressed in mathematical formulas. However, to understand what is happening during a diffraction experiment it is helpful to visulize the properties of these formulas. This is the main role of the Ewald sphere construction and understanding its meaning is important to be able to think about data collection.
Ewald sphere explanation
Using the Ewald sphere construction we can now see that only those reflections that intersect the Ewald sphere will actually diffract. Therefore, to observe all potential reflections we will have to place each individual reflection onto the Ewald sphere. This can be done by either manipulating the Ewald sphere or the reciprocal lattice. The most common method is to change the orientation of the crystal relative to the X-ray beam. This causes a corresponding rotation of the reciprocal lattice and in this manner all reflections can be passed through the Ewald sphere. The precession, oscillation, and Weissenberg methods use variations of this theme. Rotating the X-ray beam with respect to the crystal would also work in theory, but the idea of moving a hefty rotating anode around, let alone a synchrotron, is of course ridiculous. It is however possible to change the Ewald sphere by changing the wavelength and this is the principle of Laue diffraction. All these methods will be discussed in more detail following the links below.