The one you're thinking of, is for correcting a problem (usually too many nutes) and can be done at any time. The one they're talking about here - the last week or two is done to get the plants to use up all their food before harvest (better taste, better smell, smoother burning).
For the final flush, I stop using nutes and use just water the same way I do every other feeding throughout the grow, if I used 1 gallon every 4 days, I continue using 1 gallon every 4 days - I try to harvest on that 4th day, it seems to dry easier than harvesting right after my last watering.