Chiropractors diagnose, correct and stop disorders of the musculoskeletal system and look at what effects they have on the nervous system and our overall well-being. The nervous system is the controller of the whole body and is protected by the skeletal system, so if compromised, problems will manifest as headaches, asthma and colic to name a few.
Aim is to locate misalignments (subluxations) in the skeleton and to fix the problem they are causing to the nervous system. The key to well-being is the spine's vertebrae. If a unit cannot move freely it can cause irritation to a nerve, so the body parts (including blood vessels, organs and limbs) serviced by that nerve may consequently be affected. The chiropractor has to adjust the misaligned vertebra and restore normal movement within the spine... so the irritation to the nerve decreases and normality restored.
A sluggish spinal motion unit can change muscle tension and may result in muscle pain. Being on your feet for long periods greatly strains the spine, hence causing pain to appear in the lower back. Hence it is up to the chiropractor to fix it!
Summing up, chiropractic care in general is safe if operator is appropriately trained. Manipulation is relatively safe, but complications can arise, some with adverse effects resulting in trauma and contraindications.