The bit sticking up from the middle of the cap screws down, and as it does it compresses the spring.
Most riders - I'm an exception - find the front springs a bit on the stiff side, so while it looks cool this mod is of no practical value unless you also fit a softer spring. Then it's really useful.
To fit, just unscrew the existing caps and replace with these. Take the weight off the forks first though, or you'll have the spring pushing your eyeball out of the back of your head.