Underbucking is a type of log bucking. Underbucking is cutting upwards from the bottom of a suspended log.
An underbucking tool can be made using an ax driven into the side of the log so that a crosscut saw can be turned upside down.
A special tool designed specifically for underbucking makes the job of cutting from the ground upwards a lot easier. The underbuck tool is driven into the section of the log which is considered to be least likely of moving once the cut has been successfully made. The crosscut saw's backside rides along the pully that rests on the bar driven into the log. As the sawyer draws the saw backward, the sawyer may apply leverage downward and the pully will redirect the force upward into the wood.
There are many good reasons to underbuck; however most sawyers will cut from the top down, or they will underbuck half way through the log then finish the cut from the top.
Use of a single bit axe for underbucking is a preferred method for small wilderness trail crews where all gear must be packed in and out. By using the axe handle as an underbucker, the sawyer is using a tool he'd have along anyway, has a flexible support for the crosscut saw that makes it easier to maintain upward contact with the tree being cut, and will pull out easily, unlike the hammer-driven bar of a mechanical underbucker.