The external obliques assist with lateral flexion (side bending). These outer side muscles of the abdomen are underactive if you have Sway Back Posture (kyphosis in the upper body and posterior tilt of the pelvis, which leads to overactive hamstrings and underactive psoas/gluts).
How to combat Sway Back Posture?
Training the external obliques can help you stand upright and alleviate back pain, while also fighting kyphosis. Lateral flexion can be done against a wall, seated on a chair or stability ball, or lying down. As your strength increases, add resistance (weights) to the exercises you select to target your external obliques.
First: pelvic position
Before you begin exercising your external obliques, it's important that you understand the position your pelvis should be in if you have Sway Back Posture. This position is either a neutral pelvis or an anteriorly tilted pelvis (arched low back). A neutral or anterior tilt in the pelvis will allow you to turn off your overactive hamstrings and IT Band and turn on those underactive psoas muscles and external obliques.
Second: get to work
In regards to strength training for your external obliques, my suggestion would be 3-4x a week. Select three different exercises, then go through the exercises doing two sets on your non-dominant side, and one set on your dominant side. A set for me is one minute or 8-12 slow reps.
Take your time.
Inhale to prepare, and exhale on the effort pulling your belly button in towards your spine, and lifting in and up on your pelvic floor muscles (the muscles that stop your stream of pee) to contract your transverse abdominis (the deepest layer of your core) which protects your lower back. Another imperative cue to incorporate is keeping your head between the shoulders, on your spine, while keeping the neck long and simultaneously plugging down your shoulders (away from the ears).