This exercise might wreck your lumber spine.
The first issue is compression. That half sit-up position produces excessive compression of the lumbar spine.
The second issue is flexion. Because your back is not being supported by the floor, you find yourself in full flexion, where the spine is rounded. This puts the disc in a very vulnerable position.
The final issue is that the compression and flexion is combined with rotation. Imagine the pressure of juicing a lemon in a lemon squeezer… this is exactly what happens to the disc when you perform the Russian Twist.
Lastly, if you add weight to it, like holding a plate or a Medicine ball, you just make everything even more dangerous. We suggest to pick a safer exercise to work your abs and core.