When talking about core stability, strength exercises focused on the side of the torso are often forgotten. The side plank is an excellent way to train the side of the torso and all the muscles that contribute to increasing the inter-abdominal pressure including the Pelvic Floor, Diaphragm, Transverse Abdominus, Rectus Abdominus, Internal and External Obliques. The Obliques are especially heavily loaded in this exercise.
Inter abdominal pressure is vital to help to stabilise the spine since there are no major muscles supporting the spine to the front of the spinal column. Hence when the muscles at the back of the spine contract the spine will have a tendency to buckle forward into a hyper extended posture unless there is inter abdominal pressure to push back against the front side of the spine.
To perform the side plank rest one hand (or one whole forearm) on the ground as well as placing the outer side of the same foot on the ground with the other foot resting on top.
Keep the upper arm at 90-degree angle to your torso and suspend the rest of the body like a bridge/stiff-plank from the shoulder down to the foot.
Aim to keep this position stable and straight for as long as you can.
If you easily can hold the position for 30-60 sec than you could try to balance with just one foot on the ground and stretch out your free leg and arm up in the air, so that your body forms a big X lying on its side.