Yes, without that 'metal thing',
, it would be very difficult to control most horses. - The 'metal thing' is very smooth, and the horse sometimes 'leans' on it,
(it fits in a natural gap that all horses have behind their front teeth). Each horse has it's own 'bit', and they have to be of the correct size and type. When you develop 'soft hands' you can communicate through the bit with the very slightest movement of your fingers.
.....
Here endeth the lesson!