When most people perform this exercise (and all forms of rollouts), they allow their back to hyperextend and allow their body to go into a severe anterior pelvic tilt. The body should remain in a slight hollow body position the entire time (back slightly rounded instead of arched). Also, many people dominate this exercise with their upper body, largely because their core muscles are not strong enough to perform the movement effectively. For this exercise to be performed safely and and correctly, the anterior core and glutes must be working in unison. Before each rep, squeeze the glutes, take a deep breath in and surround the spine with 360 degrees of air, and brace the core. Reset before each rep.
You can make this exercise even tougher by using a thicker band, or by performing the exercise from the feet.