^^^ The first time I tried to buck hay, I worked my tail off and was exhausted in minutes. Finally the guy I was working with showed me how to use my leg, swing the bale, etc. No way I could continue lifting 100 lb bales up into the air.
If the horse has been trained for it, and most seem to be, you don't have to lift the saddle onto their back. You can kind of arc it down and swing it up an over, throwing it up and over. The way I've seen it, they will hold the front and sling it on. You could practice slinging it over a 5' corral panel to work your technique without freaking your horse.