Yes, it is very dangerous to arch your back. You could herniate your spinal discs. As with any athletic movement, it is best to leave the ego at the door and start with a low enough weight which permits you to learn proper form. I had to unlearn poor lifting techniques a couple years ago and I can tell you that it is much easier to learn it right from the beginning. Also from a training perspective, using proper form will ultimately allow you to lift much heavier weights.