His idea about practicing your pitching motion by using an underhand throwing motion is just one of many ways you can do it, although it's a very natural approach. It will also force you to slow down your change of direction because of the weight of the club. Even in a swing that develops a lot of speed, the change of direction is usually slower than you would expect.
However, I will point out something that few instructors will ever mention, even though many of them would recommend this drill wholeheartedly. Note that when Weinhart throws that golf ball with an underhand motion, he's holding the ball with the thumb, middle and index fingers of his trail hand. As I pointed out in yesterday's post, that's where you would put the grip pressure in a classic swing.
The best short game players have always based their short games on classic swing technique. Part of the reason modern pros have to practice so much is because they use two different swings in their game, whereas a classic player like Bobby Jones only had to practice a single swing technique for every shot he played. In his foreword to the book Bobby Jones on Golf, the famous golf writer Charles Price (who wrote for Golf World, Golf Magazine AND Golf Digest during his career) wrote:
It would be the most natural assumption in the world to think that during those eight years Bobby Jones did little other than play golf. In reality, Jones played less formal golf during his championship years than virtually all of the players he beat, and he beat everybody in the world worth beating. Excepting the three seasons when he journeyed either to Scotland or England for Walker Cup matches and, while there, the British championships, he spent most of the tournament season playing inconsequential matches with his father and an assortment of cronies at East Lake, his home club in Atlanta, where his interests and activities ranged far beyond matters of golf. Often, he would go for months on end without so much as picking up a club. Instead, he studied mechanical engineering at Georgia Tech, got a degree in English literature at Harvard, dabbled in real estate, and then attended law school at Emory University. Midway through his second year, he took the state bar examinations, passed them, and so quit school to practice. As a result of these off-course activities, Jones averaged no more than three months a year playing in, and going to and from, tournaments and championships. (pp ix-x)That might be something you want to consider when practicing your short game using this underhanded throwing technique. Classic technique tends to require less practice to maintain it, so try using the classic method of gripping when you pitch and see if it helps improve your feel and consistency.
Who knows? If you get good at it, the technique might even work its way into your full swing.