"BUT, as well as asking exactly what body flight is, i also asked how does one work to achieve it."
Think about which movements in ballroom you can connect with straight lines, and then where the verticies between these lines would be. Think about how that interacts with the rise and fall.
For example: lower throughout step three (both overall, and the specific offical lowering at the end of the step) with a definite direction of movement. Continue in that direction throughout step one as you commence to rise, and step two. Even if as likely you dance a CBM body rotation in step one, there will be no change in the direction of travel across the floor. As you reach the peak of the rise, your movement across the floor will be slowest - momentarialy almost zero in waltz - and so you can now lower in a new direction.
That gives you a system with the potential for managing movement energy carried between steps - it gives you times when that energy is carried uninterrupted, times when it is generated in lowering, and times when it is turned back into altitude when rising. When you are up, you can pick a new direction. When you are down you can't. Or at least, you can't if you are maintaining body flight.
Dancing with body flight is simply a means of describing a dance that takes advantage of these mechanisms, by being willing to restrict itself only to movements that work this way. If you want to change direction, or slow down, you must do it by means of rise upwards, (the general preferred method) or a compressive check downwards (to be used judicisously in only a few situations).
If you want to change direction at any arbitary time, then you are dancing without body flight. Hopefully the music playing is a tango...