I copied out below from
http://www.stihlusa.com/apparel/protect_pants.html. Yikes, and I thought the 4 hp electric I've been running was safer (from an equipment standpoint, not operator) than gas.
IMPORTANT INFORMATION
These protective garments contain pads of cut-retardant material designed to reduce the risk or severity of injury to the body parts covered by the pads in the event of contact with a rotating chain. The fibers in the pads provide extra layers of material and are designed to rip apart if they come into contact with the moving chain and to clog the sprocket and stop the chain in certain circumstances. In some contacts, the pads may stop the chain. In other contacts, the pads will resist the cutting process only for a fraction of a second. The actual degree of protection afforded will vary with the speed of the chain at the time of contact, the power and torque of the saw, the design of the chain saw sprocket and similar factors. Follow the washing instructions. Improper care may destroy cut-retardant properties of the material.
WARNING FOR ELECTRIC CHAIN SAW USERS!
The fibers will not stop the sprocket on most electric chain saws because of their constant high torque.