I think the main thing flushing does is to stop supplying nitrates so that the plant will use up all that it has absorbed to makes proteins etc, which are less harmful and harsh than nitrates. Apparently it takes plants some time to use up the nitrate that way so at least a few days of no nitrates is advisable. They reduce nitrates in vegetables the same way, cutting back on nitrate fertilizers before harvest. It's the only way to reduce them. You could still use other types of nitrogen, like urea. I don't know what the effect of phosphates are, I guess not as bad as nitrates since you never hear about too much phosphate in vegetables.