Chainsaw protective fabric works on a number of principles.
The outermost layer can be made both tough and slippery, to protect against minor damage which could compromise the filler material. Beneath this, long, loose fibres of polyester,and kevlar are laid in layers.
When a saw contacts the trousers, the outer layer is immediately cut through but the inner is drawn out and wraps around the saw's drive sprocket, locking it solid and halting the chain, limiting damage to the operator's leg. In case of an accident the fabric is pulled out of the upper and lower seam and wrap around the sprocket as described above. Protection is only given for petrol chain saws, not for electrical saws.
Trousers should be slightly baggy, so that there is give, to prevent the chain pulling the operator's leg into the chainsaw. Instead the 'baggy' excess fabric is pulled out and into the chain mechanism, thereby stopping the saw. After stopping a saw, the damaged trousers must be scrapped, having done their job. The saw must be field-stripped to remove the fibres, and to allow it to run again.
If some trousers are washed the material inside may degrade over time. As a result, these trousers should be replaced, and not washed in hot water too frequently
Likewise trousers should be free of rips and tears that may catch on a chain saw or timber when moving through a forest.
In case the trousers are damaged it is permissible to repair them only as long the inside protective fabric layers are not damaged. If they are damaged the trousers are no longer safe. They must be thrown away and a new pair obtained.