trznuti

1. (trznuti se) to flinch; to start; to twitch
   Trznuo se kad je zalupila vrata. — He flinched when she slammed the door.
   Trznula sam se iz sna od buke. — The noise jolted me out of my sleep.
2. (+ acc.) to jerk; to pull sharply
   Trznuo je ruku kao da se opekao. — He jerked his hand back as if he'd been burned.
   Pas je trznuo povodac i pobegao. — The dog jerked the leash and ran off.
3. colloq. (na + acc.) to fall for; to go for
   Odmah je trznuo na novu koleginicu. — He fell for the new colleague right away.
   Kupci su trznuli na sniženje. — Shoppers went for the sale.

In the sense "to flinch" it takes se; the variant trgnuti se is more frequent.