Placing the chocolate while his back is turned with 〜間に
Grammar Explanation: During the same time with 〜間(に)

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。歩
( が 後( ろ向( いてる間( にかばんに入( れればいいじゃん!!」- “Now! While Ayumu's back is turned, I can put it into his satchel!”
Key Points
後
( ろ向( いてる間( に marks the opening〜間
( に means that one action happens during the span of another action or state.Urushi wants to put the chocolate in his bag during the time Ayumu is facing away.
向
( いてる is a casual contraction向
( いてる is shortened from 向( いている.It describes Ayumu’s ongoing state: his back is turned right now.
入
( れればいいじゃん suggests an easy solution〜ればいい means “it would be good if…” or “all I have to do is…”.
The casual じゃん adds the feeling that Urushi has just realized the obvious move.