Hold: fans speed up to keep the gap when you step on (costs power). Fixed: fans stay put, so your weight pushes the board down.
Physics runs at 1,000 steps per second in real time: rigid-body dynamics, six skirted air-cushion compartments with variable-pitch lift fans, skirt leakage, rear thrust fans, aerodynamic and skirt drag, ground contact, water buoyancy and a 24S14P lithium-ion pack with voltage sag and current limits. Board 91 kg, deck 1.8 × 0.72 m, skirt 15 cm. Fan maps, efficiencies and the pack are engineering assumptions, not measurements.
Textures: Poly Haven (CC0). Rider: Mixamo-rigged "Soldier" character from the three.js examples, with Mixamo idle/walk clips and a physics-driven riding pose.
A 200 lb rider on a 1.8 m board that floats on a skirted air cushion and can hop about a metre. Everything you see is computed in real time.