Theoretical investigations of plasma focus discharges revealed that in magnetically driven sheaths an additional voltage is generated that is actually measured at the fast plasma focus SPEED 1 and in this case reaches values of about 10 kV. Starting from an ionizing shock wave model it is shown that this voltage is caused via charge separation in a current carrying skin layer of the current sheath by inertial forces due to sheath acceleration. Introducing this effect into circuit equations rendered possible (i) signal simulation and (ii) parameter evaluation of SPEED 1 discharges.