Nice question,
the key adjustment of this tool comes by controlling the free length of the glass fibers.
When this length is set to 1.5-2mm the glass fibers become hard enough to remove the hardest built-up corrosion.
The idea is that the glass fibers is capable to act as one knife which at the same time is incapable to damage or pollute the surface which needs cleaning with metallic particles (metal to metal abrasion).
In this setting the tools works as a pencil, and not as to was a brush.
When this length is set to 3-4mm the glass fibers become a bit softer and they are almost capable to remove soft corrosion caused by a finger print over a gold plated surface ( Memory cards).
When this length is set to 5-6mm The pencil has turn in to a brush which is capable to remove light corrosion, for example the one that is built-up in CPU sockets.
When the length of the fibers is more than 6mm it stops to act as abrasive and stops to be useful.
At this setting the fibers are capable for allot free play and it is possible to break off.
About the example with the tooth brush, just think this analogy: 1 Tooth brush hair = 8 glass fiber ones.