What he meant was that once plant converts cations and anions into cellular tissue there's no coming back! It's just basic biochemistry. To use an analogy, can you sweat your steak and salad back on the plate by drinking a GALLON of water? I don't think so! Similarly the plants don't release something they've already absorbed.
What I think you mean on the other hand is LEACHING the nutrients from the soil. Now that is possible, but it's not as simple as most of the growers think it is.
First of all current of water that passes through the soil has neutral charge, which means it collects both cations and anions, but only MOBILE ones. That means that you'll never flush out excess of phosphorus for example and it's equally difficult to flush out potassium. What gets flushed out mostly is: nitrates (nitrogen in anionic form), calcium, magnesium and sulphur... and accidentally these are the nutrients that plants needs the most in flowering, which is why leaching doesn't make any sense in the first place.
I can provide a link to an excerpt from a study that explains it well, which has been posted by me on 420.
Please learn basic botany people! It takes few hours to do necessary research! And don't spread myths, cause others will repeat them