Magnetic probe measurements show that the flux trapped into the neutral sheet radius at the early phase of the reversed field θ-pinch does not decrease monotonically but the two humps appear. The first one is coincident with the formation of the collisionless shock wave while the second one with its reflection at the center. The most probable origin of the electromotive force for these humps may be in the large rotational velocity difference between hydrogen ions and impurities'. In the first hump the velocity difference may be excited by the ion runaway (Gurevich) effect coupled with the imploding plasma. In the second hump it may be due to large off diagonal element P rθ of the ion pressure tensor created at the time of the reflection.