on one's toes

児童の読み物 "The Witches" にまたイデオムが出てきました。
I am told that The Grand High Witch makes it a rule to fry at least one witch at each Annual Meeting. She does it in order to keep the rest of them on their toes.
ここも文の前後で意味はだいたい分かりますが、一応辞書を引きます。
Dictionary.com:　on one's toes, energetic; alert; ready: The spirited competition kept them on their toes
Webster's New World College Dictionary: mentally or physically alert
ヤクザやギャングの映画にも出てきそうなシーンですね。