Port and polish

What do they do to you car when you have Port and polish done to it, how many horsepower can you expect from it and were would it help low or high rpm. Please post if you have any information seroiusly thinking of having it done.

I have a 94 Honda Prelude Vtec

They port (make bigger) the valves and polish (to make it clear of rough edges) the valve and cylinder chambers.