How to Perform the Russian Twist
The abdominals should be treated like other muscle groups, trained using a variety of exercises, set, and rep schemes. Just because you might add a little fluff, that doesn't mean direct abdominal training should fall to the wayside during mass-gain phases.
While heavy compound movements may engage the whole midsection, most elite athletes also perform isolation movements engaging the rectus femoris and/or the obliques. Whether you're a top-tier athlete or weekend warrior a strong, chiseled midsection will set your physique apart from the rest.
The Russian twist is an isolation pull exercise targeting the obliques. The iliopsoas (hip flexors), tensor fasciae latae (hip abductor), rectus femoris (quadriceps), Sartorius (inner thigh), and rectus abdominis act as supporting muscles groups during this movement. 
Supporting muscle groups assist the target muscle group in completing the movement. The tibialis anterior (shin) is the stabilizer during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving. 
The Russian twist is an excellent bodyweight movement engaging the two largest abdominal muscle groups. The rectus femoris and the obliques.
Tiger Fitness CMO Marc Lobliner details how to perform the Russian twist exercise.
How to Perform the Russian TwistYou can perform the Russian twist using only your bodyweight or with added weight like a kettlebell, medicine ball, cable apparatus, weight plate or dumbbell. This walkthrough will focus on the performing the unweighted, bodyweight-only variation, but you can apply the general form and tips to all setup variations.
Approach a slightly padded, flat surfaces like a yoga mat or exercise mat. Lie down on your back with your face up, chest high, arms, and leg straight (but not hyperextended) and in-line with the body. This is your resting position; assume this position in between sets. Keeping both legs straight, cross one leg over the other.
Take a deep breath, brace your abdominals, and begin raising your legs and torso towards each other. Stop raising both body parts when your torso and upper thighs are in the V-up position. As you raise your legs, bend at the knees so that your shins are slightly lower than parallel with the floor. This is your starting position.
Keeping your arms straight, place your hands together, palms facing inwards, and fingers pointing straight. Your arms should be pointing directly in front of you. While still holding a deep breath and bracing your abdominals, begin twisting your torso towards the left.
As you twist your torso ensure your legs don't fall or sway, your arms stay fully outstretched, and your torso doesn't curl inwards. Continue twisting and exhaling until you feel a nice stretch in the right oblique and nice contraction in the left oblique. Hold this position for 1 to 5 seconds before engaging your right oblique and returning your arms and torso towards the midpoint.
Some trainees prefer to breathe and rest for a brief moment at the midpoint before twisting towards the right. If you decide to transition twist direction without pausing in the midpoint, ensure the torso remains rigid and you're not using momentum to complete the repetition.
Continue twisting and exhaling until you feel a nice stretch in the left oblique and nice contraction in the right oblique. Hold this position for 1 to 5 seconds before engaging your left oblique and returning your arms and torso towards the midpoint.
One rep consists of twisting to the left, returning to the midpoint, twisting to the right, and returning to the midpoint. Complete for the desired number of repetitions. Avoid performing weighted variations until you can perform at 20-rep sets using only your bodyweight.
Some lifters choose to exhale while twisting their torso, at the midpoint of each rep, or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you. If you prefer performing weighted variations then experiment with holding the weight with outstretched, straight or slightly bent arms facing in-front and away from you.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets. The two most important components of any exercise are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc) but strive to improve every training session.
Russian Twist Form TipsAvoid Momentum - The Russian twist provides maximum benefits when it is performed in a controlled full-range of motion. Check your ego at the door and don't immediately attempt a weighted variation. Do not excessively swing your torso and use momentum or let your feet touch the ground (unless you want to decrease the difficulty). This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
Breathe - Far too often trainees hold their breaths during direct abdominal training. Similar to other movements, trainees should utilize natural a natural breathing pattern. Most athletes breathe outwards during the twisting portion and inwards as the torso returns to the midpoint.
Hold the Squeeze - Increase intensity by holding the twisted position of the torso for 5 to 10 seconds. Really focus on squeezing the obliques and bracing the midsection as hard as possible. This will increase time under tension and the burn in the obliques. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
References1) Griffing, James, et al. "Twisting Situp." ExRx.net. N.p., 2015.
2) Griffing, James, et al. "Kinesiology Glossary." ExRx.net. N.p., 2015.