The mechanism of switching that is taking place in a moving flux spot in a superconducting foil has been investigated in details. The structure of the current, temperature and magnetic field in the leading edge of the spot has been derived theoretically. The resulting distributions for these field variables have then been used to estimate the effective inductance and resistance of the spot. These parameters allow the determination of an equivalent electric circuit for the flux pump as an electric generator. The ratio of the inductance to the resistance of the spot decreases with the inverse square of the velocity for small velocities. On the other hand, the external current generated by the flux pump first increases with the velocity, it then reaches a maximum and eventually decreases inversely with the velocity. Design parameters, such as efficiency and dependence of the output current on the velocity for the pump, have been evaluated. These theoretical predictions were found to be verified by experimental observations which are also presented in this paper.