TheBullNotTaken: When you glob on the polish with the applicator brush it fills in all the nooks and crannies. When you use the polishing brush it removes the excess and buffs the remaining polish into a shine. The hair on the polishing brush is so fine and tightly packed that it gives it that works similar to a buffing rag for autos. For a mirror shine on black leather add a little heat and rebrush or use a very soft cloth.
Kenshin86: The brush rearranges, smoothes and aligns the waxes by physical force and frictional heat. The shine comes from a uniform and even surface of hardened wax.
TonyMatter: Sergeant told us ‘NEVER let me see you use a brush on boots’. Then we were shown how first to use a spoon, a candle, then regular spit (with a cloth).