Without a carbon tax, industry has no incentive to reduce their emissions. With a carbon tax, they have a small financial incentive to do so. Therefore they will pick the lowest hanging fruit to save some money, in the process lowering their emissions. While there is still low-hanging fruit (e.g. now, coming from where there's no incentive not to emit CO2), a carbon tax can reduce a nation's emissions without forcing large changes in how things are done.
I vaguely remember that a month or two ago, a mine in Queensland (possibly the one owned by the Indian who threatened to pull out of Australia if the carbon tax went through) worked out how to reduce their emissions by 30%.
The other effect is that the added cost of coal power due to the carbon tax/trading scheme makes gas somewhat more financially viable and renewables significantly more financially viable.
It's a very neat theory, and it's easy to see how it will affect businesses either gently (with a low price on carbon) or eventually reshape industries (with a high price).