I never knew this until someone on another forum pointed it out to me in an attempt to help me with Leo's refusing problem.

"When you are about 5/6 strides from a jump, the horse see's it in 2
When you are about 4 strides from a jump, the horse now see's it in 1
When you are 2 strides from a jump, the horse can no longer see it."

Isn't that neat? The horses are basically jumping blindly...