Dr Shettles believed deep penetration was important to give the boy sperm a head start. With less difference to travel, the boy sperm would be able to outswim the girl sperm, and find the egg.
The Shettles method claims that to improve your chances of conceiving a boy, you should have sex in positions that allow for deep penetration. It may not sound very romantic, but the closer to the cervix the sperm are ejaculated, the more likely they are to find the egg. Here are some position which can help accomplish this