I have installed numerous BC Br series coilovers on Z's... So hopefully i can help you out here.
On the rear you will set the height you want on the spring. Once you know where the height of the vehicle should be then you will tighten the 2 gold rings on the black inner piece that goes inside the rear spring OFF the car.
The rear spring should be dropped in the perch on the car and then bolt up.
In the meantime the strut should be installed but only the top bolts fastened leaving the bottom fork type piece free.
To set the pre-load you will utilize a seperate jack to raise the bottom of the metal perch the spring sits in 1/4-3/8 inch then lock the jack in place so it holds that compression.
The next point you will spin the bottom fork piece of the rear strut down until it meets up with the bolt hole it needs to be attached to. Then slide the bolt through and use the BC provided nut and tighten to spec.
Once this has been completed you will have set the pre-load.
I set at 3/8 and found it felt a little better then the 1/4 inch and always used that on future installs.
So basically the strut holds the pre-load set on the spring
Dont hesitate to ask me on here or PM me if you have any questions.
-Chad