In an AC relay (or AC contactor) the electromagnet is energized by an alternating current. The magnetic flux — and therefore the pull-in (attraction) force, which is proportional to the square of the flux — pulsates at twice the supply frequency and drops to zero twice in every cycle, at each current zero crossing. Whenever the instantaneous force falls below the restraining spring force, the armature tends to release, then re-attract as the flux rises again. This rapid make-and-break at 100/120 times per second produces the familiar contact chatter, vibration, and audible buzzing, and it accelerates contact wear.

The shading coil (a single shorted turn of copper embedded around part of the pole face) fixes this:

• The main alternating flux induces a current in this shorted ring; by Lenz's law the induced current opposes the change, creating a secondary flux in the shaded portion that lags the main flux by roughly 90°.
• Because the shaded and unshaded fluxes are out of phase, they do not reach zero at the same instant. When the main (unshaded) flux passes through zero, the lagging shaded flux is still substantial, and vice versa.
• The result is that the total attractive force never falls to zero — a continuous holding force is maintained right through each current zero crossing, keeping the armature firmly seated.

This is exactly the correct answer: the shading coil maintains a holding magnetic force during the current zero crossing. It does not delay armature release on de-energization (that is the role of the release spring and any mechanical damping), nor does it alter the relay's operating voltage rating (set by coil turns and design). It also does not physically set the contact spacing — that is fixed by the mechanical armature travel and contact geometry, not by the shading ring.