When creating bonsai, the concave branch cutter is one of the most difficult bonsai tools to substitute. When I began creating bonsai, "real" bonsai tools were not that accessible.
I would cut a branch off with heavy garden shears, and then scrape the nub down with an Exacto knife.
My first real concave cutter seemed like a miracle. It did exactly what its name says. It took less time and did a much better job. The unsightly stubs along branches and trunk lines were quickly eliminated, no "shaving" needed.
This tool comes in at least four sizes, and a number of variations.
Do not buy the two very smallest, unless you intend to work with very small plants. The largest size cutter takes most people two hands to manipulate.
The medium size (approximately 8" as shown above) is good for most pruning, unless you are working with very large branches. In that case you may even consider a saw first.Do not purchase what is called a "melon cutter" until you have more experience.
If the wood of a branch is especially hard, take a few "bites" at a time. Even though they are overall very sturdy, bonsai tools can break. Never force any tool!
Be sure you have the correct size to do the job. Forcing it can cause damage to the equipment, your tree and sometimes yourself.
A reason not to use the concave cutter is ... when cut into, the segments will die back to the next ring. Some plant examples with these segments (but not the only ones) are:
Pitchecellobium tortum, also known as Chloroleucon tortum (the Brazilian raintree) is another good example of when not to use this bonsai tool.
Working on a tree with obviously segmented branches?
You will see the rings on stems, branches or trunk. Put away the concave cutters. They often cut into a different segment than you intended.
There are several types of branch cutters. One that looks similar is the spherical knob cutter. This tool makes a much deeper cut and is not recommended as a first tool.
If you are only going to buy one "real" bonsai tool, a concave branch cutter is the one.