[Dr. Johnny Fever]

Originally Posted by petegz28

Some say it's if they dive or head first slide it keeps their fingers closed preventing a jam

This. Physical reminder that helps keep from jamming or breaking fingers. There's also a smaller mental thing that helps them (in their head at least) get into their sprint faster. What does your body do when you take off running? Get your body lower and clinch your fists which cuts on drag (minimally) but it's your natural reaction to help go as fast as possible. Already having your fists closed is like a small way of "taking a lead." Being prepared to run. It probably doesn't really help much but it's more of a good habit/muscle memory/fundamentals thing... if that makes sense.