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.
This statement explains how a leg theory attack makes it harder to score runs on the off-side by increasing the chances of the batsman hitting the ball there. To counter this, additional fielders are placed on the leg side to prevent runs and catches.