I couldn't remember exactly why but here's the answer: Shallow penetration coupled with the sperm deposited close to the entrance favors female conception because the area is more acidic, which inhibits the weaker Y sperm, according to the theory. To allow the Y sperm to reach the egg first (which supposedly moves at a faster rate), deeper penetration should be sought, to deposit the sperm at the least acidic area near the uterus opening. - That's according to the Shettles Method.
I'm still on the fence about the Shettles Method though! I don't know whether it actually works. There have been people that say it does and others say it doesn't work so it's a guessing game, but I figured it was worth trying. But now I'm really really wanting this baby so I'd rather just catch the egg than miss it again! (With the shettles method for a girl you're only supposed to BD up until 3 days before O, so that kind of decreases your chances)