Do not bend your arms. When you bend your arms, you immediately involve the triceps, shoulders and even your lats to a degree, which lessens the move’s effectiveness for the core. You may have to lighten the weight, but keep your arms as straight as possible throughout.
Rotate at your waist and hips, don't just move your arms. This exercise is made to focus on your core, make sure to keep your arms in front of you the whole movement.
Don’t stop short of the movement’s complete range of motion. Reach way up and completely rotate your torso, then bend your legs (not shown) so that you can reach almost to the opposite knee.
Keep your abs tight during the entire movement.
This exercise is great for core strength, but your form needs to exactly perfect to do it safely.
There a lot to go wrong here: you should keep the back straight as you go down and bend from the hips, as bending the back under load is really dangerous. You should also rotate from the hips and not from the back and keep the core braced and tight as you lift your arms overhead.
So basically, if you're new at training, you will probably do it wrong, there's a lot to learn and perfect here. So if you still want to do this exercise make sure to have a trainer to guide you. Don't think you can do it right just cause you read something on the internet, You don't want to get an herniated disc when you just started training.