The purpose of the shading band is to retard, in time, the portion of the flux
passing through it in relation to the flux coming out of the rest of the pole
face. In other words, the current induced in the shading band causes the
flux in the shaded portion of the pole to lag the flux in the unshaded portion.
Therefore, the flux in the unshaded portion reaches its maximum before the
flux in the shaded portion. The result is like a rotating field moving from the
unshaded to the shaded portion of the pole, and causing the motor to
produce a slow starting torque.