Allowing the spine to curve slightly can provide a better stretch on the back and neck but it suppresses the breath. It is good to perform this slight curve in the upper spine if you are not flexible.
All of the Yoga Postures can be manipulated in many different ways.
Benefits:
The hamstring is stretched (the biceps femoris, semitendinosus, and the semimembranosus) and the inner leg adductor muscles are stretched. Some of these are, the pectineus, adductor brevis and adductor longus, the gracilis and adductor magnus. The inner tendons in the knee often feel the greatest stretch. These collectively are called pes anserinus (semitendinosus, gracilis, and sartorious tendon endings). The semimembranosus muscle and tendon ending are also stretched. At the top of the legs the stretch is most often felt on the conjoined tendon of the semitendinosus and long head of the biceps femorus. The semimembranosus is also stretched at its head.
Lower back muscles and gluteus muscles are stretched due to the forward bend.
seated-splits
Yoga Teacher Tips
It is natural to want to aid students in this posture by applying a little pressure on their backs to help them down further. It is not wrong to do this but the chances of injuring a student are very high. The risk increase as the force you apply to the back increases. So push gently and get a lot of feedback as you adjust in this pose.