In a partial sit-up position on a decline bench, hold a plate, medicine ball or other weighted implement in the hands and move it from one side to the other without allowing the trunk to rotate with it. The straighter the arms, the heavier the weight, and the faster the motion, the more difficult it will be. 
 
The shoulders will rotate somewhat unavoidably—focus on keeping the sternum facing forward to minimize rotation of the trunk as a whole.


Purpose
The decline anti-rotation Russian twist develops strength and stability to maintain trunk position in the presence of rotational forces. It’s also an isometric trunk flexion and hip flexor exercise. It’s a good addition to the more common exercises that address anti-flexion, anti-rotation, and anti-lateral flexion to establish more complete trunk stability and control. Having the feet anchored in a decline bench allows more aggressive rotation and a range of possible trunk angles.
 
Programming
The decline anti-rotation Russian twist should generally be done at the end of training sessions. Sets of 8-15 reps in each direction are recommended.
 
Variations
The decline anti-rotation Russian twist can be done with any weighted implement—plate, dumbbell, kettlebell, medicine ball, sandbag, etc. Speed can be anywhere from very slow to very fast—slower speed will force longer time under tension, while faster speed will demand resistance against greater force to stop the weight at the end of the range of motion on each side.