Head-to-Knee Pose (Janusirsasana)is a good position for individuals who sit in a chair for a large portion of their day. Make sure you don't feel any pain in this position, especially in the lower-back (lumbar) region.
Before attempting to perform lumbar-spinal-flexion (bending forward at the lumbar region), it's imperative to ensure your body can come forward to 110 degrees at the hip-joint with a neutral spine. Biomechanically, following this paradgm will decrease the risk of injury in the lower back region.
Pain in the lower back can be very serious. This is a position which helps to strengthen the lower back region if practiced corectly and consistently. Practicing this correctly is very personal as all of us have different body types. You should listen to your body and make sure you practice without pain. Feeling a slight tension is how your body communicates that it is not comfortable progressing further.