I found my notes from the Yogi Breisner lecture at the BE training in Newmarket a couple of years ago. These are words of extreme wisdom and will never go out of date!
Of course at the lower levels if you have done your fitness work well your horse really should not start tiring, so this should apply mostly to the higher levels and to CCIs with their longer XC courses. But it is still well worth knowing what to do if your horse suddenly feels as if it has run out of energy.
Yogi: A rider instinctively starts pushing on a tired horse.
What to do if your horse starts feeling tired:
First, stop pushing.
Feel what the horse does. Does he slow down?
Give a couple of sharp kicks, see how he reacts.
If he picks up, good.
If he doesn’t, shorten your reins to pick him up.
If he doesn’t pick up then, smack him ONCE with the stick, see his reaction.
If there is no reaction, PULL UP, don’t jump any more!
All of this can be done within about 20 strides.
When they are this tired, you get slow-motion falls.
The first thing that goes usually when a horse is getting this tired is the JUMP. They will struggle with the width of a fence. Then they’ll lose the quality of the canter.
Wise words, to bear in mind if you ever feel as if your horse is suddenly not travelling well beneath you. Better to pull up, walk home, and get it checked out, than risk a horse falling.