By seated straddle splits, I thought initially that you might be referring to Samakonasana (Same Angle Pose/Lateral Split), in which the legs and pelvis are in one horizontal line and the hands are folded in Namaskar in front of the chest. This is a seated pose, however, with no forward bending involved. Therefore, I assume you are referring to Upavistha Konasana (Open Angle Pose). This pose is difficult for many students and requires openness in the hips, freedom in the groin, and length in the hamstrings. To master it requires patience and intelligent practice.
If your students are taking the legs wide apart, as they would in Samakonasana, and then attempting to come forward, this may be the difficulty. See that the distance is such that the fingers can grip the toes and the back remains concave in the initial seated position. A belt may be used around the feet if the students are unable to grip the toes.
There are many ways to approach and improve this pose. Indirectly, the various standing poses in general, and forward bend standing poses such as Uttanasana (Standing Forward Bend) and Prasarita Padottanasana (Intense Spread Leg Stretch) in particular, will help develop proper movement in the hips and extension in the legs. The forward bending action required in seated forward bends (Upavistha Konasana included) is more readily accessible and easier to learn in the standing poses, since one’s center of gravity is higher when standing.
Fundamental seated poses such as Dandasana (Staff Pose), which teaches the proper extension and grounding of the legs and ascension of the torso, and Baddha Konasana (Bound Angle Pose), which develops openness in the groin and inner thighs, both help develop the freedom and understanding needed in Upavistha Konasana. These and many other poses promote the understanding and uncover the hidden knowledge used to perform the seated forward bends.
More directly, when practicing or teaching Upavistha Konasana, have your students elevate the hips by sitting on two or three blankets. This will “add” length to the hamstrings and make forward bending more doable. See that the legs remain absolutely firm, extended, and grounded, and that the knees are open when coming forward. Also watch that the bending action is initiated at the root of the thighs and pelvis, not in the lower back, and that the chest and torso are extended. Otherwise, the forward bend will be incorrect and restricted. Teach your students to have the patience to hold the pose for some time, allowing the body to release and the mind to reflect.
Certified Advanced Iyengar instructor Dean Lerner is co-director of the Center for Well-being in Lemont, Pennsylvania and teaches workshop across the United States. He is a longtime student of B.K.S. Iyengar and served a four-year term as president of the Iyengar National Association of the United States. Known for his ability to teach yoga with clarity and precision, as well as warmth and humor, Dean has conducted teacher training classes at Feathered Pipe Ranch in Montana and other locations.