Shettles advice is too abstain through the beginning of your fertile period and then have sex no more than 24 hours before ovulation and not more than 12 hours past ovulation.
The reasoning is that male sperm are faster swimmers but less durable. Female sperm are slower, but more resiliant and can possibly live longer because they can handle hostile cervical mucus etc. better than male sperm can.
If you have sex say 2 days or so before ovulation, even if you then have sex on the actual day of ovulation, a female sperm could be hanging out from the first BD waiting to fertilize the egg and therefore do so before the male sperm can reach it.