Sit on the floor with your knees bent, and lock your feet under something stable.
Grasp a weight plate in both hands.
Lean backward until your torso and thighs create a v-shape.
Keeping your back straight, gently twist your torso and lower the weight to your side until you feel a mild stretch in your side or back.
Reverse the motion and lower the weight to your other side.
Keep repeating, alternating the side to which you lower the weight.
Comments and tips
Keep the movement under control. Do not let momentum build up, as you can injure your spine.
Make the weighted Russian twist easier by keeping the weight close to your body.
Make the exercise more difficult by extending the weight farther away from your body.
Instead of holding a weight plate, you can hold either a dumbbell or a medicine ball.
A more advanced way of performing the weighted Russian twist is to raise your feet a little off the floor, keeping your knees bent, and balance on your buttocks as you twist from side to side. See the video.
Your rectus abdominis only acts to stabilize your torso, which is why it has not been listed as a synergist.