In the seated version of wide leg forward bend (upavistha Konasana) you can work at stretching your hamstrings by gradually adding weight to the pose and then taking that weight away.
You add weight by lifting your arms and reaching them forwards.
So that you can support this weight you can press your legs into the floor and lengthen your spine.
Lengthening your spine in a seated forward bend tends to activate your spinal erector muscles.
To start with, practice lengthening your spine and then relaxing it while sitting upright.
Each time you lengthen focus on the feeling of reaching your ribs and head upwards. Imagine that you can continue to reach upwards.
Then when you relax, feel your head sinking forward and down and your ribs sinking down also.
To sink your ribs let your spine bend forwards.
The slower and smoother that you move, the easier it is to feel your ribs, head and spine.
Once your are comfortable with lengthening and relaxing your spine, you can add your arms.
As you reach your spine upwards reach your arms forwards. Focus on making both your spine and your arms feel long as you inhale. Then relax both as you exhale.
You might want to first practice with knees bent (below).
Then practice with legs straight (below).
While tilting your pelvis forwards, first practice pressing your legs down into the floor as you lengthen your spine.
Increase the downward pressure of your legs gradually. Synchronize it with the gradual lengthening of your spine. Then decrease the pressure of your legs in sync with relaxing your spine.
Again work at doing both actions (lengthening and relaxing) slowly and smoothly.
Position your hands close to your body.
Think of adding tension to your legs as "taking up the slack." Press your legs down first so that then it is easier to lift your hands in wide leg forward bend.
Even if you can't lift your hands completely off of the floor you may find that just the act of pulling up on your arms adds weight to the pose (you may be able to feel the added tension in your legs.)
Once you are comfortable with lifting your hands off of the floor each time you inhale, then you can reach them forwards as you lengthen your spine.
Focus on making your spine feel long. Draw your ribs away from your pelvis. Draw your head away from your ribcage.
Press down strongly with your legs.
Reach forwards with your eyes as well as your arms.
Each time you exhale first place your hands on the floor then relax your legs and spine.
With hands on the floor then relax your spine and your legs and let the weight of your body sink into the floor.
You can look down while lengthening your spine in wide leg forward bend. However make your neck feel long by reaching your ears away from your shoulders.
Likewise make your waist feel long by reaching your ribs away from your pelvis.
As you get close to the floor in this seated hamstring stretch, focus on keeping your knees pointing straight up.
You can also rest your forehead, nose or shin on the floor if you are close enough.
Once you can get a part of your head on the floor, then each time you inhale focus on reaching your chest forwards as if trying to get your belly and pubic bone to the floor also.