The primary deformity in Tetralogy of Fallot is the obstruction of the right ventricular outflow, which leads to increased right ventricular pressure and a right-to-left shunt causing cyanosis. When the child squats, the flexion of the lower limbs can reduce the return of blood to the heart, alleviating the burden on the right ventricle. At the same time, it can increase systemic circulation resistance, reducing the right-to-left shunt in the heart, thereby alleviating the child's symptoms of hypoxia.