Will someone describe exactly what happens to the XF in limp mode?

I "think" I've had it happen to me 2x and associated it with my Sprint Booster needing reset.

Basically the car wouldn't go more but 10mph until I stopped, shut the car off and restarted.

It has also happened to the new owner of my XF.

Though I don't have a great deal of fondness for him, I would like to be able to give him the valuable info.

I'd point him here, but he doesn't have a computer, is computer illiterate and you would all hate me due to his lack of social skills.

Other than the car not wanting to accelerate, there is no indication of a problem, when it happens. No dash lights, beeps, nothing.. Nada.