The most important concept behind the Shettles Method is ultimately based on the properties of the male's sperm, and the X and Y chromosomes that are carried within that sperm that will potentially fertilize the female's egg. Though it is a little known fact, not only are the X and Y chromosomes very obviously different in the fact that the Y chromosome produces a male baby and an X chromosome produces a female baby, but the essential physical properties of the X and Y chromosome carrying sperm are vastly different as well. Y chromosome sperm, which produce boy babies, are small and fairly weak, but they are extremely fast. Contrast that with the X chromosome sperm which are large, hardy and quite slow.
Many people will wonder this: this seems like fairly abstract theory, so how will that knowledge translate to help me figure out how to conceive a boy? The practical implication is in the timing, frequency and sexual positions you will use to help you conceive a boy based on your new knowledge.
First, it's important to note that in order to utilize the Shettles Method, you must determine as closely as possible the timing of the woman's ovulation. The entire method is predicated on knowing this timing, so it can't be overstated how important that is. You have a few ways to help you determine the timing: Basal Body Temperature charting, Cervical Mucus charting, and ovulation predictor tests. The simplest method is to use the over-the-counter ovulation predictor tests that are available in any drug or grocery store. These will tell you when you're within 24 hours of ovulation, so they alone can be useful to help you conceive a boy. That said, if you're looking for the full scope of knowledge on your cycle and potential ovulation, it would be smart to use these predictor tests in conjunction with one of the other charting methods, and this is especially true if you are eventually going to want to learn how to conceive a girl using the Shettles Method for some future pregnancy.
The timing of intercourse is extremely important to help you figure out how to have a boy. You already know that Y chromosome sperm is not at all hardy and will not survive in the acidic environment of a woman's body for any length of time. Therefore it is extremely important to have intercourse as close as possible to the point of ovulation as you can. If you have intercourse a few days before ovulation, many of the Y chromosome sperm will likely die and never have a chance to bring you a baby boy. Therefore, you want to use their advantage over the X chromosome sperm, which is speed. If you have intercourse close to the point of ovulation, you are giving the Y chromosome sperm the best chance to reach the egg first.
Dr. Shettles uses this same line of reasoning to suggest that the sexual position you use is also important to conceiving a boy. Since we know they are fast and it is close to the point of ovulation when you are having intercourse, you want to deposit the Y chromosome sperm as close to the cervix as possible. Therefore you want to use a position that allows for the deepest penetration possible, such as a rear-entry position.
In summary, using Dr. Shettles' method is certainly no sure-fire way to guarantee yourself a boy, but by using the methods detailed above, you are giving yourself the best chance based on the available science. It makes logical and intuitive sense based on what we know about the properties of sperm, and Dr. Shettles' own research over decades supports his gender selection method.