The equation represents all the control torques that lie on a plane that is perpendicular to . Therefore, imposing is the same as constraining the control torque to lie “behind” the plane perpendicular to . Furthermore, the control torque is obtained from (2), therefore it must always be perpendicular to the geomagnetic field. As such, the solution of this control problem must satisfy two requirements:
represents all the control torques that lie on a plane 
. Therefore, 
is the same as constraining the control torque to lie “behind”