I know trainin a horse to rear aint the same as a horse that's jumpin up on two feet straight up in the air or that's steppin backwards about to flip - but the point in the (as golden puts it) 'old egg trick'
is to smash it before the horse goes up that high- soon as that horses front end gets light and those feet come off the ground- smash the egg! Do not wait for them to go up as high as they can! That's only for the very start of the rear to stop it in its tracks.
And I never grab the neck on a rearer because the few times I had I found it made them take those wobbly steps backwards- either jump off or lean forward as far as you can with your chest pressed against the neck but never grab ahold of the neck for ballancing yourself- theres 1000 pounds of meat that's bein ballanced by two toothpicks and yes- your body weight can and will make them go over backwards! It should never ever get to this point though!!!