Insight is the process of rolling your eyeballs back round through 180° and staring directly into your brain. It is at once very painful, extremely difficult, and very frustrating; mostly due to the total absence of light in the area of your rear inner eye-ball socket.
Once you can do it you will probably realise that exactly the same effect can be achieved by moving your eyes about in the forward field of vision, and recording notable occurrences in your neural network. It doesn't hurt as much, until someone lands you with a very hard punch, simply for staring too long, or stretch the optic nerve beyond its natural limits.
Either way, thinking should be done at the time when the pain becomes unbearable.