You can use Standing Side Bend yoga pose to stretch or open the side of your ribcage, side of your waist and the side of your hip and thigh. You can also use it to strengthen these parts of your body.
If you add the arms to this side stretch you can also strengthen and stretch your shoulders as well as strengthen and stretch your latissimus dorsai muscles.
This standing side bend is designed to stretch hips, lumbar spine and thoracic spine (and the side of the ribcage.) Initially you can focus on these body elements individually.
While standing with feet hip width apart and knees straight, practice pushing your hips to one side. Pushing your hips to the left you can using your right foot. Push it to the right against the floor so that your hips move left.
Return to "center" and repeat a few times. Work at doing the actions (pushing your hips to the side and then returning to center) slowly and smoothly. Practice on both sides.
When side bending the spine one side will lengthen and the other will shorten. Bending your lumbar spine and thoracic spine to the right the right side of your waist and ribcage will shorten, the left side will lengthen or "open".
When first trying a spinal side bend focus on the open side. Try lifting the ribs away from the pelvis on the long side. Try to create space between the ribs on that side.
Next, focus on the short side. Focus on contracting the ribs towards the pelvis on the short side. Try to draw the ribs towards each other also.
Repeat both actions in isolation until you are comfortable with them.
You could try alternating actions, focusing on the short side while exhaling and the long side while inhaling.
Work at doing both actions slowly and smoothly.
You could also focus just on one action, activating it and relaxing it. In this case you can do the action while inhaling, then relax the action while exhaling.
To combine the above actions, first push the hips to the left and then bend your spine to the right.
You can accentuate the side bend of the hips by using the outer thigh on the short side and the inner thigh on the long side.
For example, bending to the right try activating the outer thigh and hip of the right leg and the inner thigh of the left leg.
A very simple action you can do while doing the standing side bend is to lengthen your spine. If you have yet to develop "spine awareness" focus on lifting your ribs on both sides away from your pelvis. Try inhaling and lifting, exhaling and relaxing. You can do the same thing with your arms reaching past your head. Try lifting the ribs, then reach with your shoulders so that your arms feel long. Then relax both actions while keeping the side bend.
As mentioned, while doing standing side bend, you can reach the arms up and over the head in the direction you are bending towards. If you have difficulty with this you can start with the hands wider than shoulder width. Work towards getting them shoulder width apart, and eventually work at touching your hands over your head.
Another option in standing side bend is to a wrist.
You can grab the top wrist and focus on straightening (and lengthening) the top arm. Or you can grab the bottom wrist.
In this case use the bottom arm to stretch the shoulder of the upper arm by straightening the elbow and pulling the bottom arm downwards.
Other options include doing standing side bend with your feet together. Balance can be trickier in this position. Stay aware of both feet so that you can keep your weight pressing evenly through both of them.
So that you exercise the muscles that you have just been stretching in this yoga pose, come up slowly and with control and while keeping your arms reaching up over your head.
The slower you do it the less likely you are to injure yourself and the more you exercise your abs.
If you are too tired to do this or you haven't yet got the strength to come up in this way, lower your arms first and then come up. You can also bend your knees to come up if your need to.