I used to have a mule that was really good at this. She would hold her breath, flex her muscles and make her belly as big as possible. You would put the saddle on and cinch it up. Then you would get on and take off. She would then let out the air and make herself as small as possible. The saddle would come loose, swing upside down and dump the rider. She would take off for the other side of the field/pasture. Sometimes, the rider(me) got dragged a bit if he couldn't get out of the stirrups. Straight out of a Western comedy.

She was an expert in unweighting the torso by moving the saddle. That mule and I had a lot of good and bad times together!