These days, when I teach people how to bb glute bridge, I have them posterior pelvic tilt and flatten out their lumbar spine. This prevents anterior pelvic tilt and lumbar hyperextension, it shuts down erectors and forces all of the burden onto the hip extensors (primarily glutes). Give it a try for a month or two and then resume normal bridging form, you'll use more pure hip extension without accompanying pelvic or lumbar motion.
