[mb6606]

He says that the right arm works, as if you were throwing a ball sidearm. But he does want a flat left wrist at impact.

Why do you guys think so many teachers tell people to flatten the shaft out on the downstroke?

Ignorance - they see many of the all time greats shifting planes (particularly to the elbow plane) and so that is what they teach.
Shift planes - just the opposite of what Homer suggested.