adverb taking the 'to' particle, taru-adjective
springing up; with alacrity; resolutely
Formal literary term describing a sudden, energetic movement or a decisive, spirited action. Often used with と (to) as an adverb.
He sprang up from his seat.
To take action resolutely.
The exact derivation is uncertain. The kanji 蹶 means 'to stumble' or 'to spring up', and 然 is a suffix forming adverbs/adjectives, so the compound suggests a sudden, springing manner.