On back hands that we have time to get around, we have the right foot forward. We do this because we are picking through the ball and this will get our bodies moving toward first for a quicker, harder throw. It also gives more room to field the ball without the left knee in the way. If your left foot is forward, you will have to take another step away from first to get your throwing side foot down.
I know this seems like little things, but I teach our infielders that we want to do the little things that will get us just a little quicker delivery to first. If we can pick up .2 to.3 seconds, we just threw the runner out by 5-8 feet instead of him being safe. And once the kids see the math, it sinks in that the little things make a difference.
baserunner with a 4.8 40time will run 25 feet/sec. or 2.5 feet every tenth of a sec.