expression, godan verb (-su)
make a serious effort; get serious; put one's heart into it
Used when someone stops holding back and starts trying earnestly, often after a period of casual or half-hearted effort.
そろそろ本気を出して勉強しないと、試験に落ちるよ。
If you don't start studying seriously soon, you'll fail the exam.
彼はここぞと
He's the type who gets serious when it really counts.