Sprung-over or sprung-under are both legitimate ways of attaching axles. Most domestic vehicles are sprung-under, and a common way of lifting them is switch the spring to sprung-over. Over or under being in reference to the axle
. My truck was built with the springs over the axle
. It seems to only be in the trailer world that people refer to this as an "axle flip".
So lifting a trailer is the same as lifting a truck, more or less. There are more and less safe ways to do it, and more and less safe heights to lift it to. At some point you're way too top heavy and the trailer's going to tip over. I've seen people here get a straight axle and
put the springs over the axle, giving them over 6" of lift. At least up to this point, none of them have posted about it if their trailer tipped over.