This is basically a means of grinding off the reinforcement material that was included to bolster the buffer tube/extension spot, and reprofiling the world to really make it resemble the first A1 region. This will likely include some welding or filling of some type. Use the images underneath for example: https://lorenzowxwtv.ampedpages.com/colt-m16a1-for-sale-things-to-know-before-you-buy-58442793