When I was a kid, the boys in my neighborhood used to play a game (we called it "step ball") where we threw a ball - usually a tennis ball - at the bottom step of a set of stairs - often the entry stairs of one of our houses. The thrower was considered the "batter." There were three possibilities (basically) when throwing the ball and hitting the step like this: if you struck the step slightly BELOW the corner of the step, the ball bounced out to the "fielders" as a "grounder" and if they caught it cleanly you had one out. If you threw the ball slightly ABOVE the corner of the step, the ball would carom off the next higher step and create a lazy fly ball which, if caught by the fielder, was also an out.
But if you precisely hit the corner of the step, the ball would take off, often so far that it went over the fielders' heads for a "Home Run." We scored it like baseball, of course, and played games of as many innings as it took to bore us to death. Lots of innings.
Well...
A couple of my formative years were spent playing this game, hour after hour, and as you might imagine, for best results you had to throw the ball sidearm.
So when I started playing Little League I - unknown to me - had a weird sidearm motion that caused the ball to curve to the right (like a screwball). My arm was also pretty strong. When I started pitching, this motion had the dual effect of making the ball look like it was going directly at the RH batters, and maintaining that left-to-right curve. It was devastating, but of course my control was to put it kindly, not good.
I had good success in Little League, Pony League, and my first year of Colt League (15 years old). When I turned 16, we got a new coach on the Colt team, and he refused to allow me to pitch, believing that my motion would ruin my arm. Until that time, no one had ever even mentioned to me that my throwing motion was odd -probably because I was having success. Nor did the coach explain to me what he thought was my throwing flaw or how to correct it. Ass-hole.
But having a car, a job (or two), a girlfriend and better things to do, I quit baseball and never looked back.
My throwing motion was very much like that of Walter Johnson in the video. It STILL feels natural to me to throw sidearm (I'll be 70 in a couple weeks), although when called upon to throw something (usually a softball) I throw overhand.