Imagine riding a bike.
When you change gear, you move the chain up or down a cog size, thus changing your gearing.
The winch line rating changes according to how many layers of cable are left on the drum.
By using a pulley and 'doubling up' the cable (running it from the winch, to the stuck vehicle or anchor point where it goes through a pulley, and then back to the winch vehucle again) you effectively multiply the capability of the winch by 2 but at the same time halve the speed the winched vehicle will move at. (In reality you lose approx 10% of the rated pull every time you run the cable back). For a really stubbornly bogged vehicle you could use 3 pulleys and 4 runs of cable to double the rating again.
A common thing is for people to hook 2 winches to a bogged vehicle and are mistaken into thinking that each winch will be doing 50% of the work.
Even if the winches are exactly matched the chances are the ratios will probably be more like 65%/35%.
In order to use 2 winches and pull equally a system containing a floating pulley must be used.