The Progressive Resistance Trainer allows for a smooth transition as the Lightning Cord Stretches and becomes tighter. Another athlete or coach holds onto the ski handle as the front athlete sprints forward creating greater resistance as Lightning Cord is stretched. The trail partner runs at a slow jog not allowing the Safety Sleeve to be exhausted. By allowing the Safety Sleeve the become fully exhausted while the front athlete is in a full sprint could cause injury to the athlete, damage the product or both. Careful supervision is recommended while using this product.