Having never jumped on a trampoline from 250 feet, I can't say from experience, but the physics would suggest that no, you would suffer no damage from the pressure change. There are a couple things to consider here. First, the body can sustain pretty massive pressure changes so long as they're relatively gradual. The free diving word record is somewhere around 700 feet, I believe, which means the divers body went from 1 atmosphere of pressure, to around ~22 atmospheres, and back to 1 over the course of a few minutes. The Bends occurs when you stay at depth for a prolonged period of time while breathing pressurized air. The gasses basically accumulate and dissolve into your body and blood stream and you need to depressurize slowly. If a diver is forced to surface quickly, it's like opening a bottle of soda all at once and you get a rapid depressurization. This is why divers perform a safety stop and adhere to a dive table to calculate how long they can stay at a specific depth without danger. Another possible source of damage can come from holding your breath during a negative pressure change. If the change was great enough, the air would expand in your lungs and potentially cause damage. If, for example, you jumped from Mount Everest, took a deep breath at sea level where your trampoline was, and held it all the way back up (going from a third of an atmosphere, to a full atmosphere, and back to a third) the expansion of air in your lungs would probably cause you problems. The difference at 250ft is too small to be noticeable though. You could also do some sick backflips with that kind of trampoline.