Strange summed it up well.
So how do they know WHEN to jump with inexperienced rider?
When horses are about 10 strides from the fence, they see it as 2 in their line of sight. Then when they get about 5 strides away, the fence merges into 1, then when they get even closer, it has completely dissapeared from their vision.
They already locked it in when they were 10 strides away.
There are 2 types of horses.
1) The horse who will do his job regardless of his riders education.
This horse has its positives and its negatives. Yes, this horse will get his rider over the fence regardless and will help his rider grow, mature, build confidence and make things enjoyable - while at the same time, wont teach the rider to be solid and functional.
2) The horse who requires his rider to be supportive and solid through their form to get them to the fence.
This is a horse only meant for a rider who has established the fundamentals and has become more confident in their riding, so that if the horse does stop due to rider error, the rider will beable to say "ok, what did I do wrong, what do I need to do to correct this" so that they become that much better the next time around.
This is a horse that will make an unconfident rider, more unconfident - so a horse that does not belong under a rider who isn't at that level where they can learn and grow from, where they wont be scared to progress forward.
Both have their place. Both teach, both educate, while at the same time they both can hinder and both can hold back a rider.
They each have their place, with the right rider on their backs.