Thanks to the cavity at the bottom, this slug can mold itself (due to the air pressure) to the barrel wall of your air rifle.
This prevents air pressure (and therefore power) from being lost when shooting with your air rifle.
As a result, you do not have to search for that one sub-caliber that fits your specific barrel and you are assured of a perfectly fitting slug.
 
Thanks to the lead pillar in the center of the cavity, the slug retains sufficient mass for high impact, flat trajectory and reduced wind sensitivity.