Some dogs do seem to just know how to turn their heads, others don't and need to be taught. How much is genetic and how much isn't, who knows. By the time the pup is ready to bite the pants, or even a tug on the leg, who knows how much it's learned just through play with the handler? And how much is the decoy doing the initial presentation, there is some very subtle stuff that can be done to get a dog to turn it's head correctly, which the handler may or may not even realize is being done.
I'll try to find a video clip, but see if you can visualize.
Dog is coming down the field. There are a couple of basic esquive moves. One, the decoy is stationary and at the last minute tries to sweep the leg the dog is targeted to out of the way. The other the decoy takes off at a 45 degree angle to the dog. The second face in the video is a good example of this type of move, although it would be better if the video was on the decoy the entire time, not the dog. There are of course other moves, but these are the most common/basic ones.
If the decoy at the last second takes off to the decoy's right, I want my dog to bite the leading leg, which is the right leg. If he's heading for the left leg by the time he gets there that leg will be gone, so I want him almost heading the decoy off, but not aiming to far ahead or the decoy can just stop and my dog will sail on past. If my dog is heading for the decoys right leg he's going to be reaching out and to his (the dog's) left to reach for the bite. If he head is turned any direction other than "eye's outside" he's going to get torqued big time if he catches that leg. Not to mention he just can't reach nearly as far to catch the leg if he's turned "eye's inside". It's possible sometimes he'll end up catching the left leg in this scenario, which would make him "eyes inside" but the goal is the right leg.
Same is true for the more stationary type of pivot.
If you watch the video of Havok, in the 3rd face he is esquived because he doesn't reach out and snag that right leg. In part because he didn't have his head turned far enough to the left to be able to reach out, it was more up/down.
This video of Chaos shows the esquive attempt, and her catching the decoy. And it's not that spectacular of a catch, visualize a faster entry, and her just barely catching him, and you'll see the type of torque I'm talking about. It's a longer video so you will need high speed connection.
http://www.youtube.com/watch?v=eDBzaunJiGk
1:17 - entry on the attack with gun