‘ Now if we put the horses' bridles into their mouths that they may obey us, we turn about their whole body also.'
The thought of bridling the body now brings to his mind an illustration, and that is that the purpose of a bridle is in order to control the horse. The whole reason for putting a bridle (i.e. the bit) into their mouths is to make them obedient. And with that bridle the experienced rider can turn the horse in whichever direction he wants it to go. And that is what the good Teacher can do, always steer himself in the right direction and keep himself under control (as a result of God at work within him). A controlled tongue will mean a controlled person.
The tongue can be a beneficial bridle or a harmful one, and the Teacher has to ensure that he makes it beneficial. The idea comes from Psalms 32:9, ‘a horse or mule without understanding --must be curbed with a bit and bridle, or else it will not keep with you'. Compare also Psalms 39:1, ‘I will guard my ways that I might not sin with my tongue, I will bridle my mouth so long as the wicked are in my presence.'
The word for ‘turn about' both here and in James 3:4 is a strong one. The bridle and the rudder are, as it were, seen to take the horses and ship and treat them as prisoners being transported. The word is used for the "transferring" or "transporting" of prisoners, or a major ‘turning about' in government. It is also used of turning men to having a better mind.