The chainsaw is a cutting device that's useful in some situations.
To start the chainsaw, enter mouselook (m) then hold PG DOWN or C and then hold the left mouse button, you will hear the starting noise and you can release the buttons (turning the saw off works the same way)
once the saw is started you can use mouse look and hold click to make a cut on any solid surface. The cut will get larger as you move away from the center. Releasing the mouse button will turn the "hole" solid and finish the cut.
Venting and Entry
The saw currently has 2 main uses.
Venting: Cutting a hole in the roof of a structure will allow smoke to escape faster and increase visibility in the structure.
Entry: You can also cut holes in walls and then click to move though the hole, this can be useful if doors are inaccessible.
Quick Reference to Use
"Bzz goes the saw.
Bzz it went...
Bzz and the roof had one big vent." - Hal Berkmans