As previously mentioned, a properly maintained engine will not need a flush. That means running full synthetic oil and keep it up on oil changes. Should have a gold hue on the inside when and if it's ever opened.
I understand some people out of necessity are on a budget and purchase used motors. I still wouldn't use the motor flush products. I cringe at the thought of dissolving carbon buildups and whatnot from a lack of maintenance, and running them not only through lifters but across the bearing surfaces. If I were to purchase a used engine to install, I would drop the oil pan and clean it till it was spotless. Most of the crud would be in the bottom. I would do a new oil pump and timing chain set. Pull the valve covers and clean as well. Then synthetic motor oil the rest of its life.
If one does use the motor flush, after draining.... I would dump in conventional oil, with a new filter, run it 15 minutes and dump it. Then refill one more time for another 15 minutes, then drain it and replace the filter, then put in a synthetic oil.