Oh, right, shields.....New idea: 50% of impact damage from cannons and missiles that is used up on the shields (I.E., hit shields with cannon round, cannon round doesn't go through) is transferred into motion on the target, sending it away from point of impact.
PHYSICS:
The purposes of our energy shields are to absorb energy, stopping attacks from damaging the hull. However, absorbing this energy is probably going to be in a fashion that spreads it out, turning it into a push rather than a punch-through situation. If you've ever watched Mythbusters, you've probably seen them use steel plates or similar protection to absorb more energy out of a gunshot. This is the same principle: Invincible shields are used to protect what's behind them, but because more of the energy is absorbed, there's more motion than if the projectile passes through the target.