To gain strength or momentum; to become more vigorous or encouraged as progress is made.
チームは連勝して勢いづいてきた。
The team has been on a winning streak and is gaining momentum.
春になって販売が勢いづいてきた。
Sales have picked up with the arrival of spring.
経験を積むにつれて、彼の演奏にますます勢いづいてきた。
As he gained experience, his playing became increasingly vigorous.