post horse, relay horse
馹 centers on a horse used for fast, relay-based communication: a post horse or relay horse that carries messages quickly along a route.
馹 combines 馬 (horse) with 日, which likely contributes a phonetic element. The character originally referred to a relay horse used in ancient postal systems.
A horse (馬) under the sun (日) gallops from station to station, carrying urgent messages as a relay horse.
For ジツ, imagine the horse galloping so fast that it seems to move in a jittery blur: jitsu -> ジツ, like a jittery post horse racing against time.