Teaching an uphill swing that promotes hitting the ball in the air. Hitting coaches that are purists (like the 2 mentioned) don’t subscribe to that method as it results in way too many pop ups, fly outs, and swing and miss. The bat doesn’t stay in the hitting zone long enough. It is especially ridiculous for scrawny young kids to be taught this way as they will never hit the ball out of the ballpark. My son had an uphill swing for years and is big & strong enough to hit the ball out of any ballpark. But in HS I can’t tell you how many times he hit a high fly ball that an OF caught with his back against the fence. It was only after working for a year with the Chicago Cubs AA hitting instructor in preparation for college that his swing flattened out and he was consistently back spinning the baseball. He is now driving the baseball and more than a few went out of the park during his first two years of college ball.
Just my opinion but I'd prefer my swing to be uphill so that it matches the plane of the pitch. I'd also prefer a swing plane that promotes hitting the ball in the air. Ground balls = outs.