The Vorsprung Luftkappe replaces your fork’s air-spring piston and increases the volume of the negative air chamber, making the spring softer in the first part of the travel (even with increased air pressure) and firmer towards the end.
There’s a version to suit most 35mm RockShox forks and 34mm or 36mm Fox forks. You might want to get a suspension workshop to fit it for you, so factor that into the cost.
I had mine installed in a 160mm-travel Fox 36 Factory GRIP2 fork by Sprung Suspension. Even with 5psi more in the spring, the Luftkappe noticeably softens the fork’s initial stroke, resulting in slightly more sag.
I raised the bar by 5mm to compensate, yet still found that traction and stability through off-camber roots was improved. The Luftkappe also keeps the fork higher in its travel when braking hard.
It makes the 36 so progressive that it’s hard to use all the travel, even without volume spacers fitted and with the compression damping fully open.
This does make it firmer on big landings though, and if you normally use volume spacers, you can remove them when fitting the kit to compensate for the added progression.