IF_I
In the ?1980's a DC10 crew were idly speculating in the cruise on where the N1 gauges obtained their signal..... They pulled a CB marked N1 for No.3 engine (on the right wing) in spite of the fact that this was prohibited unless called for by a checklist.
The engine accelerated immediately, overspeeded (oversped?), and (among other things) a fan blade broke off. This came through into the cabin, killing a passenger who was innocently enjoying the view from his window seat.
I am not for a moment suggesting that the same thing will happen if you push the A/THR pb on the ground.
What I am suggesting is .... If FCOM recommends that you do not do something, don't do it! Spend your time before departure making sure that your captain has got everything right, then I might one day choose to fly Indigo.