With a vertical fist, the entire arm is extended in one line from the shoulder through to the fist. The elbow is tucked beneath the arm as opposed to jutting outward, and the wrist is kept straight. This changes the angle at which the fist connects, and maximizes the striking surface by using the whole fist and not just the first two knuckles. Even when throwing a “rounding blow,” which is the ancestor of today’s hook, the vertical fist was used – either normally or inverted. Punching with a vertical fist provides fewer places in the arm for energy to “get lost” (like a bent elbow or wrist), and it provides more protection for the arm as a whole. The result is that that more kinetic energy is realized as force, and is distributed evenly across the fist. This protects the hand better than if the force was concentrated in one area, while still providing a powerful blow.
Professor Mike Donovan, an ex-middleweight champion, in his 1893 book The Science of Boxing, advocated using a three-knuckle landing, vertical punch. Jack Dempsey was an advocate of the three-knuckle, vertical punch, as it worked well with his "power line" theory (similar to the Wing-chun centerline theory).
However, the benefits of punching with a vertical fist are neutralized when wearing gloves. The hand is already protected so linear blows may be replaced by more circular blows like the “corkscrew” jab and, of course, the hook. These blows may be thrown with more power because they have the increased energy of momentum behind them, as well as the weight of the gloves themselves, which may weigh anywhere from 8 to 20 ounces. Additionally, because boxers need not worry about breaking their fists, they may throw punches that are more powerful Gloves, due to their size, act much like small shields around the hands, and may be used to block incoming blows. Modern boxing guards reflect this, with the hands are held close to the body to easily tuck and cover. Gloves also make getting through a modern guard with linear punches more difficult, which works to the defender’s advantage when blocking shots to the stomach or sides with the elbows, forearms, and biceps.