Talking About Horses

Straight From the Horse's Mouth
a tight rein	The rider is connected to the horse's mouth through the reins. A taut, or "tight" rein holds the horse back.
hold in check, hold in, hold back
keep a firm grip on
pull up short	A sudden sharp pull on the reins will jerk the horse's head back. Not very nice.
a free rein	Letting the reins go loose means the rider doesn't have as much control over the horse and the horse can choose how to go. it also can mean the horse has enough movement to, for instance, lower its head to pull a carriage uphill.
a loose rein
give him his head
give him some slack
high-handed, heavy-handed	Unresponsive ways of holding the reins.
chomping at the bit	The bit is a metal thing that goes in the mouth. Horses that want to go will pull on the reins. If they do it enough, they will start to foam.
foaming at the mouth
take the bit in his teeth
The bit actually rests on the bars, which are a sensitive part of the mouth just behind the teeth. If the horse takes the bit in his teeth, it doesn't feel the bit & off it goes.
curb his temper	The "curb" is a harsh kind of bit, used to get more control over the horse.
hard-bitten	A horse whose mouth is less sensitive may need a harder bit to make it respond.
take the bit in his teeth	The sensitive part of the mouth is the bars, behind the teeth. If the bit is in his teeth, the horse doesn't feel the restraint and can do what it likes.
Breaking Away
break away	The horse, who is drawing a cart or a carriage, has broken the reins and is loose
cut loose, let loose	Cut the reins to free a tangled or frightened horse -- for instance when the carriage has fallen over