Challenge page : http://www.ancientsocieties.com/probation/
The challenge text says Yes, I know this is a circular saw. Because it rotates.
On a circular saw blade, there are some teeth and expansion slots for dilatation purposes.
Here, we can see 26 spaces between the teeth (Gullets), like the 26 letters of the alphabet, and 6 expansion slots (here they are fake).
Lets have a look on the challenge’s blade.
From the bigger expansion slot to the smaller, we can read : fecows
Then, we use a Caesar Shift or Rotate cypher tool like : http://rumkin.com/tools/cipher/caesar.php
ROT 1 changes a A to a B, a B to C …
ROT 2 changes a A to a C, a B to a D …
ROT 4 changes a A to a E, a B to a F …
After a few tries, a rotate 4 gives us the answer : jigsaw
The solution page is http://stl.la/jigsaw, this page is redirected to http://www.ancientsocieties.com/enemy-combatants/