I'm not sure what the response is here. Could you please explain what do mean by "behind the guy."
In any event, here's what's what:
A base runner is always thinking FOWARD, in 90 feet increments. When he's at the plate and makes contact to the outfield, whether it be shallow, mid or deep, he's always playing the odds and probability. The odds that his ball will either drop or take a hop right in the area that may be a guessing game for the fielder or fielders going to that hit ball. So, when he's on his way to first, he's thinking and cranking his effort to see where the ball landed - if not caught. If he's three quarters of the way to first and his ball lands shallow so the fielders going after his hit converge, then he's got a slim chance of rounding the bag and making it to second. But, just in case he's careless and extends his distance from 1st too much, covering first keeps him honest and closer to the bag, maybe even a play at first. On the other hand, let's say the ball is hit mid field and he's three quarters of the way to first when the ball hits the ground, and the fielders converging on the ball hesitate just for a full second. This base runner now considers his momentum rounding first, his time in seconds to make second - usually in about 3.5 seconds, and the fielder who finally takes possession of the ball and tries to rocket that ball to second. IF the throw is better than the runner's ability to cover 90 feet in a time that is just short of the throw, and he gets caught between 2nd and 1st, having the 1st bag covered by a pitcher ( in this case) now has the chance to nail the runner.
Now here's something extra to consider. I've never been under the opinion that pitchers were fielders. I know they're suppose to be somewhat able to field, but fielding is an art all by itself. So look, if you're going to cover ANY BAG, including home plate - for crying out loud - stay out of the way of things. For first base and a runner coming back, stay to the outfield side of the bag. Give the runner - who has spikes on, enough room to have full use of the top surface of the bag. Keep you foot on that outside edge that facing right field. Home plate coverage is another issue all together and I'll leave that for another time. But, in any event, pitcher's pitch, fielders field. I know there are contrary views on that, but I've seen my fair share of too many pitchers getting banged up trying to be something they're not.