The Back Plank (or Bridge) trains your posterior core to resist flexion. Consider it a reverse front plank. This drill is a must to offset the damaging effects of prolonged sitting by activating the glutes and mobilizing the hips. Adding a mini-band above the knees increases lateral hip activation.

The proper bridging progression is as follows:

Knees Bent, Lying On Back
Knees Bent, On Hands
Legs Straight, On Hands
Legs Straight, On Forearms

You can add marches to any variation to strengthen imbalances between sides & increase hip flexor activation. My favorite variation is the Glute-Ham Walkout where you start with your knees bent and slowly walk your feet out until your legs are fully extended and then return to the starting position. Try this for 2 minutes for one of the best equipment-free glute & hamstring workouts in the game. You can even add marches to this.