Low visibility spins and spirals: difficult-to-understand explanation of false perception
Your vestibular system can only recognize acceleration. Your brain combines that with visual inputs to produce spatial orientation. (If the two disagree, you may get motion sickness.)
When you are in a sustained left bank, your vestibular system stabilizes and feels no acceleration. Without a usable visual input, your brain will convince itself that you are instead straight and level. This is spatial disorientation.
If you then look at the instruments and try to correct yourself to actual straight and level flight, the vestibular system feels that change in acceleration as starting a climbing right bank, which now feels wrong. It orders you to bank back to the left even harder to make itself feel “normal” again.
It’s not a 50/50 chance; once disorientation sets in, your body will consistently tell you to do exactly the opposite of what you should do. That’s why it’s so deadly.