Stand up straight on the leg you would like to strengthen.
Clasp both hands together in front of you.
Bend your stance leg a little to soften the knee, then reach forwards.
Bend at the hips, reaching your hands directly ahead as though going to dive.
Your elevate leg should counterbalance the movement by reaching back behind you.
Perform the movement slowly to the point you start to feel a stretch in the back of your stance leg.
Control the movement as you slowly return to the starting position.