The mechanism generating the toroidal sheet current after the sawtooth crash (observation on TEXTOR tokamak) is discussed. A possible hypothesis is presented. The crash of central pressure causes a steep gradient near the boundary between the flattened region and the unperturbed region. This sharp pressure gradient gives rise to a strong secondary current (transient Pfirsch - Schlüter current). The location, polarity, magnitude and time evolution of the current sheet are analysed. Comparison with present experimental data is made, and future necessary measurements are discussed.