When cold booting the computer the keyboard lights up, power button lights up, fans spin up, screen stays blank, this goes on for 1 - 2 seconds, then it completely turns itself off. And then repeat. After the third time it actually does not turn off, but rather the boot logo appears.
These steps also work for P52, P72 and P1/X1 Extreme. Here are the steps:
1. Plug in AC power
2. Reboot the system (we want to start from a freshly-booted state)