In the world of 1.8.9, "Reach" is the holy grail. Normally, a player can hit another from 3.0 blocks away. A "blatant" mod might set that to 4.0 or 5.0, but the server’s watchdog will snap that down in seconds.
The most "solid" versions of these mods use a technique called . Instead of telling the server your arm is longer, the mod subtly shifts the "hitbox" of your opponent a fraction of a centimeter closer to you on your client side only.
To a spectator or an anti-cheat, it looks like a normal trade. But to you, every hit lands perfectly. It’s not "Reach 4.0"; it’s "Reach 3.12." It is the distance of a single heartbeat, enough to keep an opponent in a combo forever without ever triggering a flag. The Legend of the "Private Client"