gallop
駃 centers on a horse moving at high speed: galloping. The meaning is tightly focused on this rapid equine gait.
駃 combines 馬 (horse) with 夬, which likely contributes the sound and may relate to speed or decisiveness, forming a character for a galloping horse.
A horse (馬) with a decisive, swift right side (夬) is a horse that gallops.
For ケツ, imagine a horse galloping so fast it kicks up a cloud of dust, and you hear the sharp sound 'ketsu!' as its hooves strike the ground: ketsu -> ケツ.