To lose concentration or let one's guard down; to relax one's attention and become careless.
試験中に気を抜くと簡単なミスをしてしまう。
If you lose focus during the exam you'll make silly mistakes.
スポーツの試合で最後まで気を抜かずに走った。
I didn't let my guard down and kept running until the end of the match.
会議中に気を抜いていたら、上司に指名されて答えられなかった。
I wasn't paying attention in the meeting and was called on by my boss, so I couldn't answer.