The tool consists of a wooden shaft with a weighted butt and hook-like, replaceable, U-shaped blades at the head.
The bark hack is swung much like an axe and is used to create a hatched chevron pattern ("cat face" or less frequently, a "blaze") into trees' exposed sapwood.
The weighted end helps the operator follow through with the force necessary for the hooked blade to scoop out chips of hard pine wood.
You can help Wikipedia by expanding it.The first sentence is basically correct except that the blade is straight rather than curved.
The wood hack was used up until the early 1950s and the trees were "chipped or streaked" each week during the sap flow season.