gallop
駓 centers on a horse moving at full speed: a gallop. The meaning is specific to equine motion.
駓 combines 馬 (horse) with 丕, which likely contributes the sound. The character is a phono-semantic compound for a galloping horse.
A horse 馬 beside the sound component 丕: picture a horse bursting into a gallop when it hears the sound 'hi'.
For ヒ, imagine a horse galloping so fast it lets out a high-pitched 'hee' sound: hee -> ヒ.