The Luftkappe from Vorsprung Suspension increases negative air volume in air spring forks.  This greatly improves the two main issues with air springs: a high starting preload and a weak mid-stroke.  The extra negative volume lowers starting preload to zero, creating a pneumatic top-out effect.  The reduced initial spring rate allows higher air pressure for better mid stroke support along with improved small bump sensitivity.

The Luftkappe NA2 kit also includes a low volume seal-head to upgrade the negative air volume similar to Fox 2018 Evol spec.  If you have an 2018+ Evol fork you do not need the seal-head and can buy the lower cost Evol luftkappe without.

Compatibility:
Fox 34 NA2 2016-2017:
NA2 air system only.  Does not work with TALAS or FLOAT air spring systems.
Long travel F34 with externally threaded air shaft only.  Does not fit short travel with internal threaded shaft or 2018+ with pinned air pistons.
Use EVOL version for 2018+ forks, luftkappe is the same but seal head isn't required in those.
Max 3 tokens due to reduced internal clearance.  Suggest tuning starts with 1 token.
MRP Ramp Control compatability is yet to be determined