It's partly the placebo affect that reduces pain, but ask any high school athlete about electric stim and they'll tell you it works wonders. It's the same idea of activating sensory receptors to partly inhibit or downgrade pain receptors. It's the same idea as topical capsaicin for pain relief. It's actually an irritant but that minor irritation tricks the brain into focusing less on pain so you feel better. (not completely incompatible with the meridians idea) In addition, when practitioners use electric current with acupuncture technique, it's purpose is to stimulate muscles that are too tight or overworked or compensating for another injury. When I separated my shoulder I went to 2 acupuncture sessions and it worked wonders in speeding the recovery.
That being said, acupuncture probably ain't gonna cure cancer or lower your cholesterol, but for musculoskeletal problems, and chronic pain, it's a modality that many people benefit from.