the harmonizing, mentally and physically, of two parties engaged in an activity; singing from the same hymn-sheet; dancing to the same beat
二人は阿吽の呼吸で動いた。
The two moved in perfect sync.
阿吽の呼吸でチームワークが良い。
They work in perfect harmony.
長年の阿吽の呼吸だね。
That's the sync from years together.