Reach forward to grab you toes if you can. Have your knees nearly straight but not fully extended. As you plantar flex at your ankles (point your toes), hold on and let the movement drag you forward into flexion in the back and slowly touch your chin down to your chest until you feel the neural tension stretch at the back of the legs. Then release slowly lifting the head and extending the back and at the same time dorsiflex your ankles (toes towards you) so the tension stretch remains through the movement.
Do not hold the stretch in the forward position. This is an advanced neural tension mobilisation exercise and should not be done with acute or currently active chronic lumbar disc problems where flexion is a aggravating factor.