This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
驋 combines 馬 (horse) with 發, likely as a phono-semantic compound where 發 contributes the sound and the meaning relates to a horse's action, but the exact historical development is uncertain.
A horse (馬) combined with 發, a component associated with shooting or emitting, might suggest a horse bolting or running swiftly, but the meaning is unclear.
For ハツ, imagine a horse that suddenly bolts with a sharp 'hats!' sound, like a quick exhale: hats -> ハツ.