The reference in yoga to the West means the back of the body. This includes the Achilles, calves, hamstrings, buttocks, lower and upper back, back of the arms, and the back of the neck. Unfortunately stretching of the back body seems to be synonymous with hyper stretchy hamstrings and calves.
Stretching of the back body means the entire body should receive a balanced, productive stretch that improves overall range of motion; improves daily function; and produces relief to stiff/tight muscles; and Does Not destabilize the body by over stretching ligaments, muscles and tendons which keep joints safe.
When moving into a forward fold, there should be an overall sense of stretch throughout the entire back body, not just hamstrings and lower back. The stretch should feel evenly distributed from heels to occiput (bony protrusion at the back of the skull). This way the entire body is involved in the movement, making is safer on the spine and more likely to leave the student feeling totally stretched without sacrificing form or function.
Invite the student to place a bolster under the knees and keep ankles relaxed rather than flexed. Place finger tips into the crease where top of thighs and front of pelvis meet. Fold forward pinching the finger tips between thighs and hips, and sending the ribcage slightly forward. As the body begins to move forward, also begin to slowly tuck chin, exhaling the entire way. After reaching a comfortable limit of motion, inhale and lift the body half way up, engaging muscles of back body and neck, then exhale and fold again. Repeat several times. Then hold for 10-15 seconds in the forward fold while flexing the ankles. To release, use hands to push body half way up on inhale, hold for one breath cycle, then finish sitting upright. Be sure to counter pose with a easy back extension that engages the muscles (cobra is an example) or a gentle twist to left and right.