When in this position I feel very aware of some of my lower hip muscles that I don't usually feel. I feel my spine become very extended and the twist feels nice because it moves (my spine) more than it usual.
This pose strengthens and stretches many muscles and parts of your body. For example, the legs, knees, ankles, groin muscles, spine, chest, diaphragm and shoulders are utilized. You're abdominal muscles and organs are especially helped in this asana, as it introduces a fresh circulation of blood, oxygen and nutrients to these parts.
Parivtta Parshvakonasana also aids your body therapeutically with some help in the digestion and "elimination" processes, the rid of toxins (through the twisting motion), stamina, balance and finally, spinal flexibility.
To get into this pose, start in Tadasana and lightly jump your legs out to about your arms length. Next, pivot your right foot so it creates a ninety degree angle, and your left foot so it is about forty five degrees. Make sure that both of your feet are aligned. It is very important that throughout this pose your back foot remains firmed and raised. Support this foot by keeping your kneecap "up" by engaging your quadriceps. The next step in this process is to lower your pelvis so that your front knee is directly over your front ankle to create a right angle. You should then lift your rear foot until the weight is on the ball of this foot. Next position your hips so they are facing the direction of your front leg, as even as possible. You're almost there! Finally, you are going to start to move your torso down to your front leg. After lowering your ribcage to your bent front leg, you are going to engage the spine with a twist; now gently hooking your arm in the crook of your front, bent kneecap. Extend your arm to the floor for more leverage and try to reach your navel and torso open to the sky. Finally, position your right (/arm not touching the ground) straight over your head and move your gaze towards your hand and up to the sky. You are now in Parivrtta Parsvakonasana!
In this pose you will be activating our abdominal and core muscles, quadriceps, and others.