When a muscle is stretched, the sense receptors within that muscle send information to the spinal cord to indicate that the length of the muscle has changed, in this case lengthened. The spinal cord in response sends an impulse to the muscle being stretched, triggering a contraction (tightening), it also sends an impulse to the opposing muscle inhibiting a contraction. So, stretching a muscle causes it to respond by contracting. This is counter to what you’re are trying to achieve when you stretch. This is a very basic explanation of the stretch reflex. As you can see the brain is not involved in the process at all, the stretch reflex is a spinal cord reflex.

When a muscle is contracted, the sense receptors within that muscle send information all the way to the Sensory Motor Cortex of the brain (see image below) to indicate that the length of muscle has changed, in this case shortened and also that the level of tension in the muscle has increased. Because this information has reached the brain, the muscle can be sensed or ‘felt’. It is now under your conscious control. At this point you can choose to increase, maintain or decrease the level of contraction. When pandiculating you will slowly decrease the level of contraction all the way down to complete rest. But the take home point is you illicit full cortical control over the muscle when you contract it voluntarily.   So Now you know..