I have a pretty good basis in physics so I will try an explain using that instead of canonical sources. The physics of the lightsaber itself which is espoused by b_jonas is that magnetic field controls or contains a length of plasma which some consider to be a fourth state of matter (the other three being liquid, gas, solid).
Magnetic fields generally are governed similar to the laws of electricity (as discovered by Maxwell) and the two modify each other. a moving magnetic field creates an electric field, and I think the inverse applies although the exchange is not identical (I am talking about the direction and shape of the magnetic field). Plasma carries a charge:
Wikipedia: "Heating a gas may ionize its molecules or atoms (reducing or increasing the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions"
Therefore, if the blaster is firing an electrical blast (I think plasma is a super high conductor) then when the lightsaber is swung around it will create an opposite directional field like the two poles of a magnet where the rule "like rejects like" will deflect the plasma bolt!