To avoid upper hamstring injury and get a more complete leg stretch, de-emphasize sitting bone lift/tilt.
“Lift your sitting bones” is a common instruction when moving into and deepening forward bends. While it is accurate: the sitting bones lift or tilt back when forward bending — this emphasis also puts undue pressure on the upper hamstrings and leads to hamstring insertion injuries common to forms that feature a lot of forward bends, especially when sitting bone lift is emphasized in an attempt to prevent a rounded lumbar spine.
Instead, push the entire back pelvis away from the feet and lift the pelvis off the femur heads in all forward bends. When the pelvis lifts off the femurs, greater tilt is possible, and yet does not need to be so extreme.
Find a more complete extension not only in the back of the legs but also in the front, inner and outer legs.
Yes, it is possible to create a stretch in the front of the legs, even in forward bends!
In fact, use this principle in all poses to create more space in the hip joints, which is one dimension of true “hip opening”
To find this action, have a strap handy and lie on the floor with legs bent, feet on the floor. Descend the back pelvis and feel the neutral lumbar curve. Loop the strap around the feet and extend the legs vertically, heels directly atop the pelvis. Even if your hamstring flexibility allows you to close the angle between legs and torso, hold back for now, as this is not about stretching the hamstrings — it’s about finding a line of force using gravity’s force as an aid.
Once the legs are straight and vertical, pull slightly downward on the strap, making the back pelvis even heavier. Without losing this heaviness in the back pelvis, push the feet into the strap’s resistance. Attempt to extend the front, back, inner, and outer side of the legs equally, even if the “stretch” is primarily felt in the back of the legs.
Once found, transfer this action to Ardha Uttanasana (blocks under the hands, arms straight, torso extended), then to Downward Dog (where the line of force is now diagonal, not vertical), and then finally to your deeper forward bends.
Remember to be sensitive, and to look for a more wholistic stretch — not just a sensation, but a feeling of space in the joints as well as freedom in the muscles.