Anti-tail

The antitail consists of larger dust particles left behind by the comet.

These dust particles are less affected by the Sun's radiation pressure and tend to remain roughly in the comet's orbital plane and eventually form a disc along the comet's orbit due to the ejection speed of the particles from the comet's surface.

As Earth passes through the comet's orbital plane, this disc is seen side on, and appears as the characteristic spike.

[1] The other side of the disc can sometimes be seen, though it tends to be lost in the dust tail.

The antitail is therefore normally visible for a brief interval only when Earth passes through the comet's orbital plane.