Wikipedia
Leg theory is a bowling tactic in the sport of cricket. The term leg theory is somewhat archaic and seldom used any longer, but the basic tactic remains a play in modern cricket.
Simply put, leg theory involves concentrating the bowling attack at or near the line of leg stump. This may or may not be accompanied by a concentration of fielders on the leg side. The line of attack aims to cramp the batsman, making him play the ball with the bat close to the body. This makes it difficult to hit the ball freely and score runs, especially on the off side. Since a leg theory attack means the batsman is more likely to hit the ball on the leg side, additional fielders on that side of the field can be effective in preventing runs and taking catches.
Stifling the batsman in this manner can lead to impatience and frustration, resulting in rash play by the batsman which in turn can lead to a quick dismissal.
Leg theory can be a moderately successful tactic when used with both fast bowling and spin bowling, particularly leg spin to right-handed batsmen or off spin to left-handed batsmen. However, because it relies on lack of concentration or discipline by the batsman, it can be risky against patient and skilled players, especially batsmen who are strong on the leg side. The English opening bowlers Sydney Barnes and Frank Foster used leg theory with some success in Australia in 1911-12. In England, at around the same time Fred Root was one of the main proponents of the same tactic.
Concentrating attack on the leg stump is considered by many cricket fans and commentators to lead to boring play, as it stifles run scoring and encourages batsmen to play conservatively.