It's the original 'Brain'. Specialized spec'd their bikes with these before teaming up with RockShox for the Brain. I have a Fox F100X with 100mm travel.Simply, Bump Threshold dial adjusts how hard the first bump needs to be to open the inertia valve which then allows the fork to work 'normally'.As the trail smoothens out the inertia valve closes and returns the fork to being 'rigid'.hope this helps.
