We all have our own opinions. My personal opinion is that unless you know how too sit when they rear  then you have a huge possibilty of hurting both yourslef and your horse. Also while asking for someone to give advice on teaching a hore to rear you should also ask them about teaching a down command that can be used from both the ground and seated.  True laws o gravity state that what goes up must come down but i would rather have some control over both ends. The down command is sooo much more important than the up for one main simple reason if the horse recognizes it from either the ground or seat should he for some reason be cued accidentally to go up you can bring him down even if the rider  has a slight panic fit. Therefore making things a little safer. This is only dangerous if you are irresponsible in training and execution of it. All matter's with horses  have their prospective dangers. Also each of us percieve these danger's to a different degree. Some believe teaching a horse to take a 90 degree turn is dangerous other's thing it is fun. SO to each thier own. I will not give advice on how to train your horse to go up or down as one is connected to the other. I refuse to take the chance that my advise may hurt someone. Without knowing their background with horses and more importantly having seen them actually both work and ride with horses it would be irresponsible of me to try and state how exactly to do this task. That's MHO folks, anyone who feeels like explaining to the OP  how to do these tasks feel free to. I have nothing against you advising them but be careful because sometimes a lil advice no matter how well meant or well given can go wrong. This i would suggest be taught by a trainer. Both rider and horse should be taught this not just one or the other because both have their part in the balance process.

~Ree