Thanks, Gang for being on the ball with some of the smarties, who think it's "clever" to use their electronic wizadry on board.
As a matter of fact, the "ban" period is from entering the aircraft [not from engine start], as it can interfere [seriously] with the navigation equipment [IRS..inertial reference system[s],or GNS...global navigation system[s] ] that the aeroplane uses to take us from A to B.
So please, tell them in the srongest possible way to desist, or to leave the aircraft and do what they've got to do OUTSIDE.
If they continue, please, PLEASE, come up and tell us.
For your info, I've seen the effect of one of these devices on the navigation display...thank goodness we were on the ground when it happened.