Practice without the strap first as follows:
Lay flat on your back with your left leg straight along the floor and your right leg bent and relaxed, grasping your right hand behind your knee and holding your thigh vertical (see below under ‘TESTING YOUR OWN HAMSTRINGS AND LEARNING HOW TO STRETCH THEM EFFECTIVELY)
Slide your left hand under your lower back and then slowly straighten your right leg upwards just to the point you feel your back start to press down on your hand. At this point focus on arching your lower back to maintain the space and you should feel the stretch shift into the area of your hamstrings behind your knee.
Hold this for a couple of seconds and then relax your leg back down, repeating on and off in time with your breathing, breathing out as you strighten your leg up, in as you lower your leg down.
Once you have got the hang of maintaining an arch under you lower back (this is essential), you can add the strap and increase the pressure or ‘pull’ on the stretch at the top of the movement by using your arms. Move into and out of the stretch as suggested above and always maintain a curve under your lower back.
Be careful not to pull on your calf by having the strap too high up your foot. Slightly more towards the heel is better.
To learn this and other stretches in my ‘stretching for cyclists’ workshop please get in touch to register your interest and I can keep you posted as to forthcoming workshops