The word flushing is used to describe two completely different things here.
1. Flushing with plain water the last few weeks before harvest. In fact, this may not be flushing at all, but many call feeding plain water before harvest "flushing". This particualr definition of flushing is very controversial, many recommend it, many say it's Bro science. I feed nutrients until the end and dont believe in pre harvest flushing, but thats just me.
2. Flushing the soil with twice the amount of water to soil in order to remove salt buildup or to correct PH issues. I've had lockout a few times and a good flush has helped me get back to a healthy grow. I dont like to flush, but there are times it can be very useful.