Why NVIS works by radiating signals almost straight up at elevation angles near 90 degrees so they reflect from the F (or E) region and rain back down over a circle roughly 0 to 300 miles around the station. It fills in the skip zone that normal low-angle skywave leaves empty, and it needs a frequency below the critical frequency (typically 80 m and 40 m, sometimes 60 m) since only frequencies under f-critical come back when sent straight up. Low horizontal antennas, about a tenth to a quarter wavelength high, naturally produce this high-angle pattern.
Watch out Propagation near the MUF is the opposite situation: the MUF applies to low-angle, long-distance paths, while NVIS deliberately uses frequencies well below the critical frequency so near-vertical signals are refracted rather than punching through.
NVIS = shoot straight up, come straight down. Short range, high angle, low frequency, low antenna.