A horse should be able to feel you pick up the reins before the bit does anything. That prepares him to act.
The time it takes between the rein cue and the bit or shank movement to where it engages the bit (the point at which the rider is “handling” the horse using the bit pressure) is known as the “signal”.
In the case of a simple o-ring snaffle bit, the ring will rotate slightly before the bit mouthpiece adds pressure to the mouth, thus giving more indication that a rider intends to “handle” him using bit pressure. The “signal” is better than with a more fixed bit.
If the shanks of a curb-action bit are adjusted at a proper angle (usually about 45 degrees), the horse will have time to realize that the shank is moving and prepare for the body cue before the bit is actually engaged. If properly trained, he will anticipate the request, and only a very light cue will suffice to direct his movement.
A bit with a looser curb and a longer shank will have a longer “signal” time. Additionally, when a bit is balanced so that its rein loops hang a little forward of center when not engaged by the reins, it takes just that fraction of a second longer for the reins to take up the slack, which lengthens the signal time. A bit balanced this way is an advantage for quick release of pressure and reliable neutral position. When the reins are dropped, the bit immediately swings forward to its balanced position and releases the pressure. (see Release Training)
Pat Perelli understood that a novice or inexperienced trainer needed to give a horse as much “signal” as possible. He recommends that the trainer may benefit from adding a light-weight extension from his reins to the bit (slobber straps or a light-line of about 6″ length) , extending the amount of slack that it takes to put actual pressure on the bit and ensuring that the bit is released at the quickest possible moment.
A short signal time is to be used only by the most experienced riders.