I have a large Koi pond. I have always been puzzled as to how anti-chlor works. You can put as little as a half cup of anti-chlor in the pond and add 1000 gallons of water and it is all treated. I do not understand how the anti-chlor can be dispersed evenly enough to treat all the water. I use a product that "removes" chlorine, chloramines, and heavy metals. I can only guess that it must neutralize and not "remove" but I am still at a loss as to how.

The dechlorinator reacts with the chlorine compounds to render them harmless. It's called a redox reaction. The Chlorine part of molecules are oxidised, and the dechlorinator is reduced.

You only need a small amount because these compounds aren't a very large constituent of the water. You also don't need to completely dechlorinate the water - only enough to make it safe for your pond life.

I hope this helps - please ask if you have further questions!