The pivot leg is more than just the leg that supports the body as it turns-twists-collapses and drives in the direction of the pitch. (basically). This leg is an integral part of the body’s balancing system as the stride leg stretches out and plants itself followed by the turning of the hips…… then the upper body uncoils and the shoulders rotate exchanging the glove shoulder with the pitching shoulder. All of this mind you while the body is descending down a sloping surface.
So, some pitchers have an unsolicited response from their balance sensors/mechanism to hold back the pivot leg. Some pitchers extend their pivot let straight out behind them upon finishing their delivery, while others simply drag their leg along or just inches off the surface of the mound. Tom Seaver was one who used this technique. In fact, if you watch some of his video as he progressed in age – skill, you’ll notice he alters this style slightly depending on his pitch selection on any given day.
By the way, you’re a pitcher, right? Try this technique for yourself and see if it actually balances you off, makes the delivery process more stable, and/or lets you fit into the mound(s) better. However, I’m only suggesting that you try this posture, just as a teaching aid so that you’ll learn more about why you sometimes do things while your pitching. I am NOT a proponent of this format nor do I support it’s deliberate use as fundamental. But, sometimes the human body makes an unsolicited response in reaction to its environment – static or otherwise.
An extreme example to the contrary – upsetting the balance between movement and momentum can be see in the video footage of Hall of Fame great Bob Gibson, when he was with St Louis. His motion from the windup was so pronounced - tilting noticeably towards first baseline that his pivot leg literally kicked up and spun around in a half-circle. A very intimating thing for a batter to be looking at, I can assure you.
In any event, some pitchers – like Gibson, because of their pitch selection, or their forceful drive forward, will actually whirl around either to their glove side or their pitching side, or, will dive downward with a short but pronounced stutter-step (like skipping), mostly because of the pivot leg’s contribution or lack there of the balance afforded.
Again, as a pitcher – go out to the field and try various pivot leg postures. You would be amazed at how important this leg is to your delivery, accuracy, and finish form.