Background. Rugby requires explosive strength in the lower body, agility, and stability, with eccentric actions being key in performance and in the prevention of injuries. Although isoinertial training has shown promise in enhancing eccentric strength in other sports, its effects on rugby-specific skills remain understudied. Objectives. This study aimed to examine the impact of a six-week isoinertial strength training program on improving kinetic stability, feinting ability, and kicking power in youth rugby players. Materials and methods. Forty male athletes (age: 15.2 ± 0.6 years) were randomized into two groups: an experimental group (EG; n = 20) performing isoinertial training twice weekly for lower extremity strength, and a control group (CG; n = 20) who continued with traditional training. Pre- and post-intervention evaluation included the Y-Balance Test (kinetic stability), Fitlight-driven feinting tests, and radar-measured kicking speed. Results. The EG showed significant improvements in the Y-Balance score on the right leg (+4.5%, p < 0.001) and left leg (+3.9%, p < 0.001), faster feinting reaction times (−3.6%, p = 0.005), and enhanced kicking power (+9.8%, p < 0.001) when compared to the CG, which had no substantial changes (p > 0.05). Conclusions. The study concludes that isoinertial eccentric training enhances rugby-specific performance metrics, likely mediated by improved neuromuscular control and eccentric strength. Therefore, these findings support the inclusion of isoinertial training in youth rugby conditioning programs to enhance agility, stability, and power.