From the side of the knee one can better appreciate the position of the patella (kneecap) in relation to the underlying femur. The area of contact between the undersurface of the patella and the groove of the femur is the patellofemoral joint.
The position of the patella relative to the groove of the femur changes as the knee is bent. In the straight knee the patella lies above the groove and can be freely wiggled from side to side. As the knee is bent the patella engages with the groove of the femur and it should not be possible to wiggle it from side to side. In some people with a very shallow groove, it is possible that the patella may jump out of the groove either partially (which is called subluxation) or fully (which is called dislocation). These are both abnormal circumstances.