carry things on one's back
揹 centers on carrying a load on one's back, using the body as support.
揹 combines 扌 (hand) with 背 (back), suggesting the action of carrying something on the back with the hands supporting it.
The hand radical 扌 beside 背 (back) shows a hand helping to carry a load on the back.
For ハイ, imagine carrying a heavy load on your back and letting out a sigh: 'hai...' The sigh cues ハイ while you carry the burden.