As a lowering spring is a compromise, they have to be able to stay on the strut on full drop. To get this most spring makers put a soft bit in the spring that fully compresses on load. In other words those coils with the plastic on are compressed together, the plastic is to stop them rubbing the paint off and corroding away. In the picture of the spring above, there are 7 coils, but with weight on you only have 4 coils. Lowering springs are just a crude form of lowering. To do it properly you need shorter shocks with the original length of spring.