Electromagnetic acceleration is one of the electrical accelerations; it exhausts propellant by the electromagnetic force. MPDT (Magneto Plasma Dynamics Thruster) and PPT ( Pulsed Plasma Thruster) are one of this type thrusters.
Furthermore electromagnetic acceleration thrusters are classified by some types, whether steady acceleration or unsteady (pulsed) one and whether the magnetic field is a only self induced field or includes applied field.
In those, PPT is belonged to a pulsed and self induced field type accelerator. Mechanics of acceleration is shown in following diagram. First, by the main discharge, the energy stored in a capacitor is delivered to the electrodes, and high current is conducted in the circuit and plasma. Secondly, magnetic field is induced around the current; the field between electrodes is in the direction penetrates the screen from back to front (I had no idea of a better expression of this direction). Thirdly, the interaction of this field and the current, which conducts between electrodes from bottom to up, induces the Lorentz force , and the plasma is accelerated in the right direction. This phenomena would be more readily understood when you consider the two currents which are parallel and in opposite directions.
Although quite large current is required to accomplish effective electromagnetic acceleration, pulsed operation of PPTs enables to generate high instantaneous current (10 kA level) in short bursts (10us). Hence effective electromagnetic acceleration can be obtained.