Will only work for cases where the examiners check like this:
1. Let you in to the exam hall in normal mode by making sure light is not blinking on your calculator (normally without this exploit the light should blink both in turned on mode and turned off mode if in Presstotest, so they do not usually check turned on/turned off mode)
2. Then let you begin exam once you put it into Press to test mode by checking to make sure light is blinking
3. So in summary, this hardware trick hides the fact that a calculator is already in Presstotest mode when the calculator is turned off. Normally, the light would blink regardless of its turned on or turned off status if the calculator is in presstotest mode.

Only for educational purposes, I do not condone anything illegal. Don't cheat on your exams.