Please follow the instructions carefully when you are doing forward bends. For example, in the half moon–padahastasana series, the instructions are to bring your torso on your thighs, chest to the knees, no light space anywhere between the torso and the thighs. If you try to straighten your legs without having this first part accomplished, you can create pain in your lower back. Also keep in mind the instruction “360 degrees stretching.” What that means is to make the stretch even throughout the whole back side of the body. (See previous question for more details).
You also want to follow these guidelines in separate leg stretching. The teacher says, “Suck in your stomach and bend your upper body down from your lower spine towards the floor.” It is difficult to suck in your stomach unless you exhale, so make sure to exhale as you fold forward. When you suck in your stomach, it supports the whole pelvis to roll forwards with the rest of your spine–this eliminates strain in the lower back. Working carefully, mindfully, allowing your breath to flow with your movement will help you to gradually deepen your experience of the postures.