Start on your back. Lie flat on the floor with your arms extended straight up toward the ceiling. Bring your legs up so your hips and knees are both bent at 90-degree angles (tabletop position).
Flatten your back. Engage your core by actively pressing your lower back flat into the floor. You should not be able to slide a hand underneath your back. This is the most crucial part of the exercise.
Extend opposite limbs. Slowly lower your right arm backward toward the floor while simultaneously straightening and lowering your left leg. Hover both just an inch or two above the ground.
Maintain core tension. Keep your lower back glued to the floor. If your back starts to arch, do not lower your arm and leg as far.
Return and switch. Use your core to pull your arm and leg back to the starting position. Repeat the movement on the opposite side, lowering your left arm and right leg.