I think you just need to aim at the center of the shield. The better your Jousting skill means the further off the dead center you can be and still win (might also affect how much it bounces around, I don't remember). Of course your opponents jousting skill comes into play in that equation as well.
It's odd that no one has done an in-depth analysis of the gameplay for DotC. It's not like it's not a well known and popular game. I've looked and all I ever see are general reviews.