Rugby is a sport played mainly in Europe and the Southern Hemisphere. It's similar to American football, with plays that involve running with the ball in hand, although only the ball carrier can be tackled in rugby. Scoring is also similar, but in rugby, the ball must be grounded in the end zone. Significant differences between the two are that in rugby, you cannot pass the ball forward, nor can you wear protective equipment.
Like football, rugby has a number of specialist positions. The scrum half is one of the most important. Rugby is split between forwards and backs. The forwards, also known as the pack or scrum, are normally the bigger, stronger players who get stuck in, fight for the ball and push up the pitch. The backs are the faster, more skillful players who spread across the field, waiting for the ball to come their way so they can run at the opposition and score. The link between these two groups is the scrum half, generally the smallest player on the pitch, who takes the ball from the forwards and sets the play in motion.
As the link between the forwards and the backs, the scrum half effectively dictates how the team plays. He can control the pace of the game when the team is attacking by choosing when to keep the ball rolling with the forwards and when to pass the ball to the backs to open up the running game. He can also choose to run with the ball or kick the ball himself.
The scrum half must be tactically aware. Because he can influence the game, he needs to take responsibility and make split-second decisions. If you're a scrum half, you can study video footage and graphics so that you're aware of when you can make certain plays, but no amount of studying will replace actual experience. You should be aware of how your backs are set up behind you and how the opposition is lined up. If there's an overlap, you should try to pass the ball to that side or kick into space to start an attacking move.
Scrum halfs need to focus on agility. You need to be able to squeeze into tight spaces to get to the ball at the scrum or during a ruck. You also need to work on passing the ball quickly and accurately. A common drill is to pick a ball off the ground and pass it in one movement, often with an added dive to minimize the time and distance traveled by the ball as you come off the ground. The dive pass is just one of the moves favored by the scrum half. Whether you are kicking, passing or running from the base of the scrum, you need to move quickly, as you will become a target as soon as you lay hands on the ball.
The scrum half also has defensive duties, although typically as one of the smallest players on the pitch, he is given less responsibility. When the opposition is in control of the scrum or at a breakdown, he will be lurking at the edge of the scrum, level with the ball, looking to tackle the opposition scrum half or block the ball as soon as he picks the ball from the base of the scrum.