The Russian twist is a type of exercise that is used to work the abdominal muscles by performing a twisting motion on the abdomen. The exercise is believed by those who practice it to build explosiveness in the upper torso, which may help in sports such as tennis, swimming, baseball, track & field, hockey, golf, lacrosse, or boxing.
The origin of this exercise is not exactly known.
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To perform the Russian twist, first, one should sit on the floor with knees bent as in a "sit-up" position. The feet should be kept together and slightly above the ground or put under a stable surface. The torso should be kept straight with the back kept off the ground at a 45 degree angle. Arms should be held together away from the body in a straight fashion and hands kept locked together like a ball or one can hold a weight to increase the difficulty. Next, the arms should be swung from one side to another in a twisting motion, with each swing to a side counting as one repetition. The slower one moves the arms from side to side, the harder the exercise becomes, working the abdomen that much better. When moving one's arms during the exercise, it is crucial to not stop between repetitions or else one will lose the effect of working the abdomen. Constant breathing in and out during the exercise is important as one should not hold one's breath.
The Mason twist is a more advanced (difficult) version of the Russian twist. It is performed in the same manner, except the legs are straight and the feet are held off the ground (in the V-sit position) for the duration of the exercise. 
Other Russian twist
The phrase Russian twist can be used to indicate a special case that applies to Russia. An example is a case where smugglers bought grain alcohol from the US and smuggled it into Russia, disguising it as windshield-wiper fluid or cleaning solvent. 
- The Russian Twist: Men's Fitness
- Russian Twist
- A Smuggling Operation With a Russian Twist, New York Times, August 19, 2000