Japanese by Example
Learning through examples in manga

Placing the chocolate while his back is turned with 〜間に

Grammar Explanation: During the same time with 〜間(に)

それでも歩は寄せてくる » Volume 4 » Page 91

Urushi prepared chocolate to give Ayumu for Valentine’s Day but is too nervous to give it to him. After the two play shogi during their club, Ayumu turns away from Urushi as he puts the shogi table away.

うるし:
「今(いま)。歩(あゆむ)が　後(うし)ろ向(む)いてる間(あいだ)にかばんに入(い)れればいいじゃん！！」
"Now! While Ayumu's back is turned, I can put it into his satchel!"

Here, 間(あいだ)に marks an inclusive overlap, where the moment Urushi places the chocolate in Ayumu’s satchel takes place during the span of time when Ayumu’s back is turned toward her.