The Roman Emperors did not have that. What your'e thinking of was Roman statesman during Triumphs. Triumphs were essentially a huge parade that was considered a huge honor for Military success. Most men could only expect to ever receive one Triumph in their career if at all. The General would ride on a chariot pulled by four horses and a slave would stand behind them whispering in their ear "Remember you are only a man".This was to remind them that no man was bigger than the Republic. This is only for the Republican Era, im not sure if the practice continued into the Imperial Era