trimming reduces friction which reduces fletching wear and increase speed. The new Biscuits trimming is not nearly as important or effective as on the older ones. Over trimming can result in decreased holding power so you need to be careful. Fat aluminums will tolerate more trimming while the skinny pultruded carbons really won't take any trimming before they start to "slip between the bristles" CAP has a diagram on there website, it's under the tuning for finger shooters section.