The construction takes three pivots. A median line runs from the first through the midpoint of the second and third, and parallel lines run through those two pivots, producing a fork-shaped channel projected forward.
The claimed tendency is that price gravitates toward the median line and reacts at the outer rails. In practice the pitchfork is a disciplined way to draw a trend-channel with a rule, which is a genuine improvement on freehand lines.
It remains fully dependent on which three pivots you choose, and changing any one of them redraws everything. Like all geometric tools it describes the shape of past movement; treating the projected lines as future levels assumes the trend's geometry persists, which is exactly what is in question.
Related: trend-channel, trendline, swing-high, swing-low, chart-clutter