The principle of an energy shield is that it converts the energy of an incoming attack into another, less aggresive form, since energy cannot be destroyed or removed, it can only be converted or exchanged. This is why there is often a 'flash' as an impact strikes the shield, which converts the energy of the attack into light.
However, it does depend a lot on how the shields are designed. Possible outcomes could be...
- shields stop low power attacks
- shields have a chance of stopping an attack, no matter what the power level
- shields reducing the incoming attack
- shields work against physical/energy attacks only/differently
- shields work against 'fast' attacks and not 'slow' torpedoes/attack craft
- shields provide an overall cover, or have to be focussed (see Robotech/Macross, for example)
And there are a lot more, and these dont include Eldar shields which are not really shields at all but ECM.
I always assumed that Imperial shields were fairly primitive defences, working against all 'fast' attacks (weapon batteries, lances and other similar attacks), that they were 'all or nothing' - so they didnt reduce the power of an incoming attack, but either blocked it completely or did nothing. I assume that the mechanics of shields were drawn from the Titan shields in Epic.