Yep, that's me; and yes, I know what the cue to slow down the horse is -- lean back and use both reins. And yes, you can stop the horse by doing "slow down" three times...
But that's not a way to stop the horse. If you are going full gallop and need to stop, you want full stop now cue, not three slow down cues.
Now, I knew some horses that were actually very good at stopping, and yes, there's huge difference between stop now and slow down to full stop. Cue those horses were trained to was "whoa"...
So I tried teaching that cue to young stallion here, and it does not really work. Or rather... it works a bit too well.
I know many horses where "whoa" means slow down so I sometimes utter it when I want to just slow down... and then the horse comes abrubtly to full stop. What is worse, many other words trigger same response -- I guess they are too similar for stallion's ears.
There must be some reasonable cue, that is impossible to mistake for the horse, and unlikely to be given accidentally by the rider... unintended full stop is almost "and now climb back to the horse" event... but what is it? For now I know "whoa" is neither :-(.
(And for the record, I probably could teach horse to do full stop on something completely crazy -- like hand touching his tail -- he's learning almost too quick.)