Sometimes people have difficulty doing a bridge exercise.
It can bother their neck, they can over use their back muscles, it can make their hamstrings cramp, or the other side can compensate.
The issue with this is that the bridge is a very important exercise for many injuries.
Activating and strengthening your glutes is imperative to recovery.
This is great varation for people that may be experiencing these problems.
If you need to activate your glute better, try this exercise.