From Longman Dictionary of Contemporary Englishbe the brains behind/of somethingbe the brains behind/of somethingDEVELOPto be the person who thought of and developed a particular plan, system, or organization, especially a successful one Danny’s definitely the brains of the project. → brain
Examples from the Corpus
be the brains behind/of something• Bakewell senses that Rudd was the brains behind the fraud, but it was the brothers who were condemned to death.• Builtin computers are the brains behind them.• Maybe it was them who built the fences and he was the brains behind the organization. 