expression, suru verb (irregular)
turn one's back to
Describes physically turning so that one's back faces something or someone. Often used when deliberately facing away or positioning oneself with the back toward a direction or object.
彼は私を背にして立っていた。
He was standing with his back to me.
窓を背にして座ってください。
Please sit with your back to the window.
背を向ける also means 'to turn one's back,' but often implies a deliberate act of turning away, sometimes with a nuance of rejection or ignoring someone. 背にする focuses more on the resulting state of having one's back toward something.
Compound of 背 (se, 'back') + に (ni, particle) + する (suru, 'to do'). Literally 'to make (something) into one's back,' i.e., to position oneself with one's back toward something.