Meir, thank you for the great explanation.
Let me elaborate further: I want to integrate the mechanism, the relays and a
push button (click me) to make this happen.
The push button will be installed underneath the brake pedal in a way that it activates under hard braking (and not each time that I push the brakes). I will install it as precise as possible that the brake lever pushes the switch whenever it is pressed all the way down. Whenever I take my foot out, it retracts automatically.
As I suspected (and you confirmed), the ECU in the Boxster controls the operation of the motor and cuts power at a precise time delay (4-6 seconds...). But in your case, if there is a fault between your relays and ECU, that means your motor has no way of stopping once you push your manual switch. Otherwise, the ECU does not control the timer... and there is something we don't know here.