  We study the dynamics of a spin-dependent quantum dot system, where an
unsharp and a sharp detection scenario is introduced. The back-action of the
unsharp detection related to the magnetization, proposed in terms of the
continuous quantum measurement theory, is observed via the von Neumann
measurement (sharp detection) of the electric charge current. The behavior of
the average electron charge current is studied as a function of the unsharp
detection strength \gamma, and features of measurement back-action are
discussed. The achieved equations reproduce the quantum Zeno effect.
Considering magnetic leads, we demonstrate that the measurement process may
freeze the system in its initial state. We show that the continuous observation
may enhance the transition between spin states, in contradiction with rapidly
repeated projective observations, when it slows down. Experimental issue, such
as the accuracy of the electric current measurement, is analyzed.
