When does it become the second baseman's responsibility to cover first base in baseball?

When does it become the second baseman's responsibility to cover first base in baseball?

Alan Jaeger (Pitching Coach / Founder) gives expert video advice on: What is a 'second baseman' in baseball?; What are the defensive responsibilities of a second baseman in baseball?; Where should a second baseman in baseball position himself on the diamond? and more...

It becomes the second baseman's responsibility to cover first base, number one, in any bunt play where he knows ahead of time that the first baseman's job is to crash home plate, meaning that he has to come get the ball. Secondly, if there's a runner on first and there's a ground ball hit to first baseman in a double play situation, if the first baseman has to field the ball away from first base, by the time he throws it to second base to get the lead runner, he won't have time to get back to first base, so the second baseman's job is to make sure he gets to first base to cover for the first baseman.