Why does it work?
Toner powder does have some advantages in that it is black. This is useful as it blocks out sunlight from the plant/weeds
Waste toner powder is a dense powder which means it sticks to the plants
It does not wash away very quickly - for my photographs to highlight that the plants were actually dead I had to use a vacuum to remove the powder.
It makes no difference with or without ferrous oxide as some powders contain this and some do not.
Works well no matter the temperature. 
Simply brush away once plant is dead or just leave.
NO IT IS NOT A WEEDKILLER by it self it does need simple additives but these can be simply and cheaply added anywhere in the world.

contact angus.carnie@virgin.net for more details or visit www.powderrecycling.co.uk