This exercise works your obliques where as most core exercises just work your mid section (abdominals) it also strengthens your back which in turn helps with maintaining good posture.
There is no equipment needed for the Russian twist unless you wanted to add a weight so they can be done anywhere there is a bit of space.
Detailed Workout Plan
Lie down on the floor placing your feet either under something that will not move or by having a partner hold them. Your legs should be bent at the knees.
Elevate your upper body so that it creates an imaginary V-shape with your thighs. Your arms should be fully extended in front of you perpendicular to your torso and with the hands clasped. This is the starting position.
Twist your torso to the right side until your arms are parallel with the floor while breathing out.
Hold the contraction for a second and move back to the starting position while breathing out. Now move to the opposite side performing the same techniques you applied to the right side
Repeat for the recommended amount of repetitions.