Seated forward bend or paschimottanasana is a safe pose to practice regularly. The primary precaution as in standing forward bend is being careful with the lumbar spine. The pose is a deep forward bend and one must be careful not to strain the lumbar musculature in addition to herniating discs from posterior pressure.
To safely practice the pose, you’re working on stretching the hamstrings: semimembranosus, semitendinosus and biceps femoris to be exact. You can bend your knees as much as you want but slowly engage the quadriceps and lift the kneecaps. This beginning of lengthening the legs and straightening them will give you the tension to need to begin opening those deep tight hamstring muscles.
The strap shown allows one to focus on opening the calves and achilles tendons. The additional grip with the hands also allows more pull, a pull that’ll slowly allow you to lengthen the hamstrings and begin lengthening the spine. Always remember in forward bends, the pressure is in the backs of the legs, no sharp pain is felt behind the knees and you allow the hamstrings to open slowly as you breathe. Only then do you allow the spine to fall open and begin tractioning the spine.