Why In ordinary multi-hop propagation the wave bounces between the ionosphere and the ground, and each ground reflection costs several dB of loss. Chordal hop occurs when tilts or gradients in the ionosphere refract the wave back up at a shallow angle so it travels chord-like from one ionospheric refraction to the next without ever touching the ground. Skipping those lossy ground reflections is why chordal-hop paths, common on transequatorial and long-path openings, deliver unusually strong signals over very long distances.
Watch out Propagation across the geomagnetic equator describes transequatorial propagation, which often uses chordal hops but is defined by geography, not by the missing ground reflection; departure from the great circle bearing describes skewed or skew-path propagation.
Chord = a straight line inside the circle: the signal stays up in the ionosphere and never touches the ground.