FWIW, having my share of back problems, I decipher this as meaning because of a spine injury, he/she cannot keep their arms elevated without support, so they lean forward with their wrists or forearms resting on their knees, and when moving hands left and right on the keyboard is required, they do so by moving their knees left and right, all to avoid lifting the arms from their support.
Painkillers might explain the rest
When my lower back is bad, lifting my foot from the gas to the brakes is not easy.