The left arm and the right arm behave differently throughout the swing. Straight left. Bent right. Lead arm. Trail arm.
A terrific way to understand how both function is to hit some balls one handed. See video below. It's harder than it looks.
Nick Faldo hits thousands of balls this way with his right arm as he was revamping his swing.
"There isn't enough daylight in any one day to practice all the shots you need to"- Ben Hogan This is a great quote and always punches home the reminder that to be good- really good- at the game of golf there is no substitute for hard work.
When Nick Faldo was grinding away working on his swing alteration back in the mid 1980's he religiously practiced by making one arm swing with his right arm only and hit a bunch of balls with a split hand grip.
A natural golf swing is a loosely thrown around term. Let me make this clear. With the right information ANYONE can learn to play good golf.
Learning new swing patterns and feels can be difficult. The reason being motion dictates motion so things begin to happen on a grander scale.
Golfers try long and hard to alter their swings for the better. Some have success. Many fail.