The following contribution was complied by Peter Steinberg (of Penn State University) [The text was all written by him except the final section "Who to watch to learn more"].
The fullback on a rugby is the last line of defence and the extra man in attack. He must be able to kick, be a good open field tackler, catch well under pressure and have the pace to come into the back line. Fullback is one of the back-bone positions on the team (Hooker, No.8, Scrum-half, flyhalf and full back).
The full back is often the extra man in attack. As he does not start in the back-line alignment he can "float" and come in at any spot and change the angle. The angle of the fullbacks entry into the line is very important. Before he recieves the ball he should not be looking at the passer, but at the defence and try to find a gap. Trust the passer to put the ball into the right place! The angle can be especially devestating against a sliding or drift defence. If the FB can take a flat pass, while running against the movement of the defence he will almost always break the gain line. He must comunicate with the ball carrier, telling him where he is e.g. short/long.
The FB usually comes into the line inside the winger and outside the outside centre. This is the best place. Sometimes the FB may come into the line outside the wing, but this is a lot easier to defend. The FB has less space and the defence can just slide along one player.
The FB is part of a mini-team, along with the 2 wingers, called the back three. These players must cover for each other at any time of the game. On offence, if the FB goes into attack the blind side winger must move into a more central position and cover the FB area. If the blind-side winger goes into attack, the FB should move slightly to the blind side to cover the wingers area.
This is the most important role of the FB, the last line of defence. In complex defences used today (drift, slide, scrape etc) the FB should not have a player to cover in the attacking backline. This allows the FB to concentrate on covering the kicks of the opposition. Again the "back-three" must work together to cover the whole field.
As the ball is passed along the backline, the FB shadows the ball in a deep position. The blindside winger must also follow across the pitch taking the FB place. If there is a break, the FB will have to make an open field tackle. As in all defence the FB should take away the decision making from the ball-carrier. To do this the FB should force the BC to run in one direction, do not come up square on the BC so they can go either way. Take one shoulder and make the BC run away from you and if possible his support. Then take the angle and tackle from the side.
Any chance that the back three get to run the ball from a kick should be used. If the ball is caught on the full from a line kick the wingers and the FB have time to make a decision. As soon as the ball is kicked the back-three should go towards and support the reciever, telling him how much time he has, and where potential space is. Once the catch is made then the decision to counter is done using three cues:
1) Time - if caught with the ball straight away, the BC must stay on his feet and fight for position to allow his support to get back. DO NOT go to ground. Remember the opposition can hit the ruck/maul directly while his support must run back then enter from their own side. If not caught, then you have time to look for space and read the defence.
2) Support - the BC should not try to counter without support. If he has time to catch and look up, at least the other 2 members of the back three should be there. The IC should also work to get back in support. If there is no support then a kick is the favored option (as long as you can kick accurately). Put the ball infront of your forwards, and do not forget to run to put them onside.
) Cover Defence - This is another tricky one. The kick is used in Intl. rugby alot when the BC has support and time because the cover defence is very good. Look for space, and remember that the space need not be in front of you, it could be wide for your support. Do not try to run through a team that is coming up together, use the boot.
Kicking - there are 2 kicks you can do. A line kick (spiral) into space. Look for the wingers to be out of position as they follow up the kick. If it bounces you have done you job. It is very good if you can get it to go into touch. Rugby is a game of Territory (7's is a game of posession). The other kick is the "up-and-under". This is a high kick that you can follow and put pressure on the ball immediately. Try kicking this point-down onto your laces, you do not get as much height but you greatly improve your accuracy. This is used primarily in the opposition half and is a more attacking kick than spiral kick.
Counter moves - First their must be support, and then there must be a recoginition of where the space is, then you must try to maintain that space. Most of the cover will come from the forwards, so the space will be away from them. To maintain this space it is good to have the ball carrier run towards the forwards and then switch with the blind-side wing. This freezes the forwards and maintains the space wide. Another move is to have the supporters loop around the BC. This gives the BC options, and allows him to decide which way to go. It is important that all the players keep the ball alive early in the counter, do not run away from your support! Communication and a mutual understanding of these principles will allow a back three to work together.
If you want to learn more about playing fullback then you probably should start by watching the ways that the top players have done it.
Top class fullbacks have basically come in 3 varieties.
There are the physical players, strong in defence, and hard straight runners in attack. Classic fullbacks in this mould include JPR Williams who played for Wales from 1969 to 1981, any tape of Wales playing from the mid to late seventies should give you a glimpse of JPR's many attributes. More recently Scottish international Gavin Hastings filled much the same roles. Try to get a video of some of his early performances (1986 to 1988), and probably more easy to find are any from Scotland's games throughout 1995.
The second type of fullback are those whose game is based on their speed and elusive running, as well as their lines of running. The two classic modern examples are New Zealand fullback John Gallagher, who can be seen at his best in the All Black World Cup winning side of 1987; and Serge Blanco who played for France from 1980 to 1991.
The final type of fullback are the utility type players. Good basics, and an ability to read the game lie at the heart of their play. Often what tips the balance in these players' favours is their goalkicking abilities. Perhaps the best example of this style of fullback is Australia's Marty Roebuck who played in their 1991 World Cup winning side.