These ‘inserts’ are made to fit inside the ‘neck’ of the S3 Titanium front pipes and are designed for the 125cc GGPRO, BETA EVO and SHERCO to increase bottom end power.
What they do is slightly decrease the diameter of the front pipe. They are an ‘interference’ fit and need to be ‘pressed’ into the neck of the front pipe.

S3 Titanium front pipe – ‘insert’