It is the removal of the command that constitutes a reward and reinforcement for learning. The removal is most effective in the first second after the correct response. Beyond three seconds is usually ineffective. To decreasing sensitivity to stimuli, keep presenting the stimuli at an intensity just below that required to produce a significant reaction. Remove the stimuli as soon as the horse ignores it.
Timing is similar to that required to increase sensitivity to commands.
Keep increasing the strength of the stimuli slowly. The horse will learn to ignore a quite strong stimuli. This exercise is particularly useful in teaching the horse to accept and ignore anything that frightens him such as weeds, bushes, saddle pads, white plastic bags, etc. A primary mode of communication between horses is touch. Horses will spend hours grooming each other.