That's just it. One cannot explain it. One can only appreciate it. The ingredients that go into being a genius producer aren't all tangible. It's not science. So just saying vary so and so every second or fourth bar etc won't make you great. What makes you a great producer is knowing what sounds magical and what doesn't. Even more important what sounds sound magical together, forget vocals. They're just sprinkling of gold flakes on a gold bar. Hope I've made some sense and sorry for the late reply
