Depends, i think most people refer to flushing as only giving the plant water in the final week/week in order to get rid of the minerals/fertilizers and cause the plant to eat itself reducing the chloryphll and making a smoother tastier smoke (if this is accurate i havent a clue and you will see alot of experienced growers on both side of the fence)
Now some techniques like "make it rain" call for running tons of water through your plants/pots. I dont know if its works or not but i imagine as long as your plants are established and you let it dry out it wouldnt do much damage (once again growers on both sides of the fence)
I would LOVE to see side by side mass experiments with good genetic clones using each theory and seeing the result on yield/quality/taste.
And yes overwatering really becomes an issue when it is done repeatedly and your soil isnt allowed air thus drowning your plants