When we draw the human body we can reduce it to a simple stick figure, with linear limbs attached to a linear torso. Yet the human body is far more complex and intricate than that. Even though our torso and limbs appear linear, the composing muscles and fascia (connective tissues) are not stick straight.
Take the back of our upper legs - the hamstrings - for example. They are a group of muscles that start from the thigh bone (femur) and the hip (the iliac crest). They all run south and insert at and below the knee, but some connect into the outside of the lower leg (the fibula) while others connect into the inside (the tibia).
In other words, the hamstrings flow in a similar but not identical direction. They are not aligned parallel like train tracks.
Functionally, this muscle arrangement allows the hamstring to pull and bend the knee more effectively. But why and how does this matter in terms of stretching (and strengthening)? When you perform a common hamstring stretch by keeping the leg straight, bending the body forward, and holding the stretch statically, you are not engaging the fibers evenly. You are stretching as if you are a stick figure and all the muscle fibers are parallel.
You can improve on it by repeating this static stretch at different angles, but that is very time consuming. On the other hand, if you stretch and move simultaneously, you can efficiently and evenly engage all the muscle fibers in the target group. In Essentrics when we stretch our hamstrings we can do so while standing or using a barre/chair. After we gently fold the body forward, we rotate the working leg in the hip joint, reaching each of the hamstring muscles. Moving while stretching is a powerful way to develop muscle awareness as we feel the stretch shifting from one part to another. This dynamic approach is used throughout the routine, which explains why our entire body feels so balanced after an Essentrics workout!