It's the same as your standard lowering spring setup except the rear can be adjusted based on the length of the ink you will need to install. Essentially you are just fooling the rear into thinking the vehicle is at a set height.

Anyways my understanding is the H&R kit will drop more than advertised however it will still be a conservative drop without compromising ride quality. I personally really enjoyed the Eibach front setup on my X5. It rode very close to stock sport. The rear springs didn't offer much drop but with rear airbags you wouldn't have to worry about that.