To suddenly become motivated, get into a particular mood, or switch into a focused mode (figuratively 'a switch is turned on').
会議の直前になって彼のスイッチが入ったらしく、次々にアイデアを出し始めた。
It seemed his switch flipped just before the meeting; he began offering ideas one after another.
音楽を聴くと仕事のスイッチが入ってはかどる。
Listening to music turns on my work-mode and I get more done.
このゲームを始めるとすぐにスイッチが入って、夜遅くまでやってしまう。
Once I start this game, the switch flips and I end up playing late into the night.