In dynamic breaking, the traction motors become generators. It is MUCH harder to turn traction motor in generator mode, and since the wheels are geared to the traction motors, they don't turn well either. In order to get rid of the electricity, its run through the reistors and converted to heat energy, with some elctircity being stored if needed, or it can use the heat to keep the internal stuff warm.
With a locomotive and train, you'd obviously need alot of resistors to disiapate so much electricity produced by the momentum of the train forcing the locomotive forward.
trains without dynamic breaks use just regular air brakes. when a train does have Dynamic breaks, it increases the life span of the Air brakes and wheels since not all the energy is going into the pads and wheels ( and therefore causes less wear and tear), and the energy is released as heat from electricity.