Warm-ups help the tissue to loosen up and allow the tissues to expand better. Warm-downs, which should actually should be cool downs, should involve something that allows the tissue to contract, like cool water. This helps in the healing process for ligament strain. I remember going through this type of warm-up, cool-down routine for Physical Therapy because it promoted good circulation and also helped with ligament healing. If you're just doing jelqing though, it should help with better circulation which could help with length gains. You may want to consider stretching though, as that the best way to gain length.