Butt-Ups

Muscles worked

Filled = primary · Outlined = secondary

Source: Free Exercise DB (public domain)

Begin a pushup position but with your elbows on the ground and resting on your forearms. Your arms should be bent at a 90 degree angle.
Arch your back slightly out rather than keeping your back completely straight.
Raise your glutes toward the ceiling, squeezing your abs tightly to close the distance between your ribcage and hips. The end result will be that you'll end up in a high bridge position. Exhale as you perform this portion of the movement.
Lower back down slowly to your starting position as you breathe in. Tip: Don't let your back sag downwards.
Repeat for the recommended amount of repetitions.